Multi-piece solid golf ball

ABSTRACT

The invention provides a multi-piece solid golf ball having a solid core, an inner cover layer and an outer cover layer, which outer cover layer has numerous dimples on a surface thereof. The ball is characterized by combining an inner cover layer and an outer cover layer that are each formed to specific material hardnesses and thicknesses with dimples which satisfy specific conditions. This multi-piece solid golf ball is able to substantially reduce the distance traveled by the ball when struck at a high head speed, while at the same time holding down the decrease in distance when struck at a low head speed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser.No. 12/757,761 filed on Apr. 9, 2010, the entire contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-piece solid golf ball having asolid core, an inner cover layer and an outer cover layer, and havingnumerous dimples on a surface of the outer cover layer. Morespecifically, the invention relates to a multi-piece solid golf ballwhich substantially reduces the distance traveled by the ball whenstruck at a high head speed (head speed is sometimes abbreviated belowas “HS”) while at the same time undergoing little reduction in distancewhen struck at a low HS.

With recent advances in golfing equipment such as balls and clubs, golfballs have come to travel increasing distances. For this reason, to keepplay fair, strict rules have been adopted which establish, in the caseof a golf club, for example, the size of the head and the length of theshaft. Similarly, restrictions have been placed on certaincharacteristics of a golf ball, such as its size, weight and initialvelocity, so as to limit excessive ball travel of the sort that wouldresult in a loss of fair play.

The distance traveled by a golf ball is generally held down by limitingthe initial velocity. However, in such cases, both at high head speedsand low head speeds, the distance traveled is often reduced in about thesame ratio. As a result, such balls have significant drawbacks for lowHS players.

As another approach, various golf balls have been disclosed which, byoptimizing the dimples on the surface of the ball, lower the flighttrajectory and hold down decreases in distance.

For example, JP-A 05-103846 describes a golf ball in which the dimplediameter, dimple depth and number of dimples have been optimized. JP-A10-043342 and JP-A 10-043343 disclose golf balls in which the amount ofdeformation by a ball when compressed under a load of 100 kgf has beenoptimized, along with which the dimple diameter divided by the dimpledepth has been set to a value of from 10 to 15 or the dimple spacevolume as a proportion of the total volume of a hypothetical sphere werethe surface of the ball to have no dimples thereon has been set to from0.7 to 1.1%. JP-A 2000-107338 discloses a practice golf ball in whichthe ball weight and diameter have been optimized.

In addition, JP-A 6-142228, JP-A 7-24084, JP-A 9-10358, JP-A 11-253578,JP-A 11-253579, JP-A 11-319149, JP-A 2000-70408, JP-A 2000-70409, JP-A2000-70410 and JP-A 2000-70411 disclose golf balls having a cover with arelatively soft inner layer and a relatively hard outer layer.

It is therefore an object of the present invention to provide a golfball which can achieve a superior distance in a low HS range whileholding down the distance traveled in a high HS range.

SUMMARY OF THE INVENTION

The inventors have conducted extensive investigations in order toachieve the above object. As a result, they have found that, in amulti-piece solid golf ball composed of a solid core, an inner coverlayer and an outer cover layer, which outer cover layer has numerousdimples on a surface thereof, by specifying the thicknesses and materialhardnesses (Shore D) of the inner cover layer and the outer cover layer,and also the size relationship between the material hardness of theinner cover layer and the material hardness of the outer cover layer; byspecifying, for the dimples formed on the surface of the outer coverlayer, the number of dimples, the dimple surface coverage (SR), thedimple volume ratio (VR), the dimple types, the average dimple depth,and the dimple diameter-to-depth ratio (DM/DP); and by maintaining thecoefficient of lift CL at a Reynolds number of 70,000 and a spin rate of2,000 rpm at a specific ratio or more of the coefficient of lift CL at aReynolds number of 80,000 and a spin rate of 2,000 rpm, synergisticeffects arising from dimple optimization and the suitable hardnessrelationship between the inner cover layer and the outer cover layermake it possible to substantially reduce the distance traveled by theball when struck at a high HS while at the same time holding down thedecrease in distance when the ball is struck at a low HS.

That is, unlike conventional methods of lowering the ball initialvelocity or core initial velocity, the golf ball of the presentinvention is able, by combining low-trajectory dimples with the internalstructure (multilayer structure) of the ball, to substantially reducethe distance traveled by the ball when struck at a high HS while at thesame time holding down to the extent possible, relative to the reductionin distance on high HS shots, the reduction in the distance traveled bythe ball on low HS shots. As used herein, “distance” refers to the totaldistance traveled by a golf ball, including both the carry and the run.

Accordingly, the invention provides the following multi-piece solid golfballs.

[1] A multi-piece solid golf ball comprising a solid core, an innercover layer and an outer cover layer, which outer cover layer hasnumerous dimples on a surface thereof, wherein the inner cover layer hasa thickness of from 0.8 to 3.0 mm and a material hardness, in terms ofShore D hardness, of from 10 to 60, the outer cover layer has athickness of from 0.7 to 3.0 mm and a material hardness, in terms ofShore D hardness, of from 45 to 62, and the material hardness of theouter cover layer is higher than the material hardness of the innercover layer; the dimples number at least 250 but not more than 500, havea surface coverage (SR) of at least 70% and a volume ratio (VR) of atleast 1.06%, are of at least three types of mutually differing dimplediameter (DM) and/or dimple depth (DP), and have an average depth of atleast about 0.18 mm and an average diameter-to-depth ratio (DM/DP) ofnot more than about 23; and the ball has a coefficient of lift CL at aReynolds number of 70,000 and a spin rate of 2,000 rpm which ismaintained at 60% or more of a coefficient of lift CL at a Reynoldsnumber of 80,000 and a spin rate of 2,000 rpm.[2] The multi-piece solid golf ball of [1] wherein, letting Da representdimples having a diameter of at least 3.7 mm and Db represent dimpleshaving a diameter of less than 3.7 mm, the ratio (total number of Dbdimples)/(total number of Da dimples) is at least about 0.005 but notmore than about 1.[3] The multi-piece solid golf ball of [2], wherein the dimples Dahaving a diameter of at least 3.7 mm account for at least about 75% ofthe total dimple volume.[4] The multi-piece solid golf ball of [1], wherein the value obtainedby subtracting the material hardness of the inner cover layer from thematerial hardness of the outer cover layer (outer cover layer materialhardness−inner cover layer material hardness) is, in terms of Shore Dhardness, at least 5 but not more than 50.[5] The multi-piece solid golf ball of [1], wherein the dimples have anaverage edge angle of from 11 to 17 degrees.[6] The multi-piece solid golf ball of [1], wherein the proportion ofdimples having an edge angle of from 12 to 16 degrees is more than 70%of the total number of dimples formed on the surface of the ball.[7] The multi-piece solid golf ball of [1], wherein the value obtainedby subtracting the inner cover layer material hardness from a surfacehardness (Hs) of the core (Hs−inner cover layer material hardness) is,in terms of Shore D hardness, greater than −10 and less than +10.[8] The multi-piece solid golf ball of [1], wherein the value obtainedby subtracting the outer cover layer material hardness from a surfacehardness (Hs) of the core (Hs−outer cover layer material hardness) is,in terms of Shore D hardness, at least −15 and not higher than +5.[9] The multi-piece solid golf ball of [1], wherein the ratio ofdeflection by a sphere composed of the solid core encased by the innercover layer (inner cover layer-encased sphere) when compressed under afinal load of 1,275 N (130 kgf) from an initial load state of 98 N (10kgf) to deflection by the solid core when compressed under a final loadof 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), whichratio is represented as (inner cover layer-encased spheredeflection)/(solid core deflection), is from 0.82 to 0.92.[10] The multi-piece solid golf ball of [1], wherein the ratio ofdeflection by the ball when compressed under a final load of 1,275 N(130 kgf) from an initial load state of 98 N (10 kgf) to deflection bythe solid core when compressed under a final load of 1,275 N (130 kgf)from an initial load state of 98 N (10 kgf), which ratio is representedas (ball deflection)/(solid core deflection), is from 0.72 to 0.79.[11] The multi-piece solid golf ball of [1], wherein the ratio ofdeflection by the ball when compressed under a final load of 5,880 N(600 kgf) from an initial load state of 98 N (10 kgf) to deflection bythe ball when compressed under a final load of 1,275 N (130 kgf) from aninitial load state of 98 N (10 kgf), which ratio is represented as (600kgf deflection/130 kgf deflection), is from 3.2 to 3.7.[12] The multi-piece solid golf ball of [1], wherein the core has acenter hardness (Hc), a surface hardness (Hs) and a cross-sectionalhardness (Hm) at an intermediate position between the core center andthe core surface which, in terms of Shore D hardnesses, satisfy thefollowing conditions:

Hm−Hc=0 to 7,

Hs−Hm=11 to 25, and

Hs−Hc≧16.

[13] The multi-piece solid golf ball of [1], wherein the core has acenter hardness (Hc), a surface hardness (Hs) and a cross-sectionalhardness (Hm) at an intermediate position between the core center andthe core surface which, in terms of Shore D hardnesses, satisfy thefollowing condition:

Hs−Hm>(Hm−Hc)×3.

[14] The multi-piece solid golf ball of [1] wherein dimples Da with adiameter of at least 3.7 mm have an average diameter of at least 3.7 mmbut not more than 6 mm, and dimples Db with a diameter of less than 3.7mm have an average diameter of at least 1 mm but less than 3.7 mm.[15] The multi-piece solid golf ball of [1], wherein dimples Da with adiameter of at least 3.7 mm have an average depth of from 0.05 to 0.5mm, and dimples Db with a diameter of less than 3.7 mm have an averagedepth of from 0.05 to 0.3 mm.[16] The multi-piece solid golf ball of [1], wherein dimples Da with adiameter of at least 3.7 mm have an average volume of from 0.8 to 3.0mm³, and dimples Db with a diameter of less than 3.7 mm have an averagevolume of from 0.2 to 1.5 mm³.[17] The multi-piece solid golf ball of [1], wherein dimples Da with adiameter of at least 3.7 mm have an average diameter (Dm) to averagedepth (Dp) ratio Dm/Dp of from 7 to 25, and dimples Db with a diameterof less than 3.7 mm have an average diameter (Dm) to average depth (Dp)ratio Dm/Dp of from 10 to 30.[18] The multi-piece solid golf ball of [1], wherein the cover is formedof a material comprising:

(A) a thermoplastic polyurethane material, and

(B) an isocyanate mixture obtained by dispersing (B-1) an isocyanatecompound having as functional groups at least two isocyanate groups permolecule in (B-2) a thermoplastic resin that is substantiallynon-reactive with isocyanate.

[19] The multi-piece solid golf ball of [1], wherein the cover is formedof a material comprising:

(D) a thermoplastic polyurethane, and

(E) a polyisocyanate compound.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a cross-sectional view showing the internal structure of amulti-piece solid golf ball according to an embodiment of the presentinvention.

FIG. 2 is a schematic view illustrating a dimple used in the presentinvention.

FIG. 3 is a top view of a golf ball showing a dimple pattern (I) used ona ball in an example of the invention.

FIG. 4 is a top view of a golf ball showing a dimple pattern (II) usedon a ball in an example of the invention.

FIG. 5 is a front view of a golf ball showing a dimple pattern (III)used on a ball in a comparative example.

FIG. 6 is a front view of a golf ball showing a dimple pattern (IV) usedon a ball in a comparative example.

FIG. 7 is a front view of a golf ball showing a dimple pattern (V) usedon a ball in a comparative example.

FIG. 8 is a front view of a golf ball showing a dimple pattern (VI) usedon a ball in a comparative example.

FIG. 9 is a cross-sectional view for explaining the edge angle of adimple.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The golf ball of the invention is a multi-piece solid golf ball having asolid core (sometimes referred to below as simply the “core”), an innercover layer and an outer cover layer. The outer cover layer has asurface with numerous dimples formed thereon. By combining an innercover layer and an outer cover layer, each formed to specific materialhardnesses and specific thicknesses, with dimples which satisfy thesubsequently described specific parameters, the distance traveled by theball on shots taken at a high HS can be substantially reduced whilesuppressing a decrease in the distance traveled by the ball on shotstaken at a low HS. As used in the present invention, “high HS range”refers to a range of about 50 to 60 m/s and “low HS range” refers to arange of about 30 to 40 m/s.

The internal structure of the inventive golf ball G is described.Referring to FIG. 1, the ball G has a three-layer construction composedof at least a core 1, an inner cover layer 2 encasing the core 1, and anouter cover layer 3 encasing the inner cover layer 2. In this invention,the inner cover layer 2 and the outer cover layer 3 are sometimesreferred to collectively as the “cover.” Numerous dimples D are formedon the surface of the outer cover layer 3; these dimples D satisfy thespecific parameters of the invention. It should be noted that, althoughFIG. 1 shows a three-layer construction arrived at by forming a core 1,an inner cover layer 2 and an outer cover layer 3, any of these layersmay be optionally formed as a plurality of two or more layers withoutdeparting from the scope of the invention. For example, the core may beformed as a plurality of layers.

The core in the invention may be formed using a rubber compositioncontaining, for example, a base rubber and also such ingredients as aco-crosslinking agent, an organic peroxide, an inert filler, sulfur andan organosulfur compound. The base rubber of the rubber composition ispreferably one composed primarily of a known polybutadiene.

In the present invention, an organosulfur compound may be optionallyblended in the base rubber in order to increase the rebound of the core.When an organosulfur compound is included, the amount included per 100parts by weight of the base rubber may be set to preferably at least0.05 part by weight, more preferably at least 0.1 part by weight, andeven more preferably at least 0.2 part by weight. The upper limit in theamount included may be set to preferably not more than 5 parts byweight, more preferably not more than 4 parts by weight by weight, andeven more preferably not more than 2 parts by weight. If the amount oforganosulfur compound included is too small, a sufficient corerebound-increasing effect may not be obtained. On the other hand, if toomuch organosulfur compound is included, the core may become too soft,resulting in a poor feel when the ball is played and a poor durabilityto cracking on repeated impact.

The diameter of the core, although not subject to any particularlimitation, may be set to from 30 to 40 mm. The lower limit value ispreferably at least 32 mm, more preferably at least 34 mm, and even morepreferably at least 35 mm. The upper limit value may be set topreferably not more than 39.5, more preferably not more than 39 mm, andeven more preferably not more than 38.5 mm.

The core has a center hardness (Hc) which, although not particularlylimited, may be set to, in terms of Shore D hardness, preferably atleast 25, more preferably at least 28, and even more preferably at least31. The upper limit also is not particularly limited and may be set to,in terms of Shore D hardness, preferably not more than 50, morepreferably not more than 45, and even more preferably not more than 40.If the center hardness is too low, the rebound may be too low, resultingin a less than desirable distance, the feel at impact may be too soft,or the durability of the ball to cracking on repeated impact may worsen.On the other hand, if the center hardness is too high, the spin rate mayrise excessively, possibly resulting in a less than desirable distance,or the feel at impact may be too hard.

The core has a surface hardness (Hs) which, although not particularlylimited, may be set to, in terms of Shore D hardness, preferably atleast 45, more preferably at least 48, and even more preferably at least51. The upper limit also is not particularly limited and may be set to,in terms of Shore D hardness, preferably not more than 70, morepreferably not more than 65, and even more preferably not more than 60.If the surface hardness is too low, the rebound may be too low,resulting in a less than desirable distance, the feel at impact may betoo soft, or the durability of the ball to cracking on repeated impactmay worsen. On the other hand, if the surface hardness is too high, thefeel at impact may be too hard or the durability to cracking on repeatedimpact may worsen.

The core has a cross-sectional hardness (Hm) at an intermediate positionbetween the core center and the core surface which, although notparticularly limited, may be set to, in terms of Shore D hardness,preferably at least 30, more preferably at least 33, and even morepreferably at least 36. The upper limit also is not particularly limitedand may be set to, in terms of Shore D hardness, preferably not morethan 55, more preferably not more than 50, and even more preferably notmore than 45. If the cross-sectional hardness is too low, the reboundmay be too low, resulting in a less than desirable distance, or thedurability to cracking on repeated impact may worsen. On the other hand,if the cross-sectional hardness is too high, the spin rate may riseexcessively, resulting in a less than desirable distance, or the feel atimpact may be too hard.

As used herein, “center hardness (Hc)” refers to the hardness measuredat the center of the cross-section obtained by cutting the core in half(through the center), and “surface hardness (Hs)” refers to the hardnessmeasured at the surface of the core (spherical surface). In addition,“cross-sectional hardness (Hm) at an intermediate position between thecore center and the core surface” refers to the hardness measured at apoint midway between the core center and the core surface on the abovecross-section. Also, “Shore D hardness” refers to the hardness measuredusing a type D durometer in general accordance with ASTM D2240-95.

In this invention, the value Hm−Hc obtained by subtracting the corecenter hardness (Hc) from the cross-sectional hardness (Hm) at anintermediate position between the core center and core surface, althoughnot particularly limited, is preferably set to, in terms of Shore Dhardness, from 0 to 7. The upper limit in this value may be set to, interms of Shore D hardness, more preferably not more than 6, and evenmore preferably not more than 5. The lower limit may be set to, in termsof Shore D hardness, more preferably at least 2, and even morepreferably at least 3. If the above value is too large, the durabilityto cracking on repeated impact may worsen. On the other hand, if thisvalue is too small, the spin rate may rise excessively, as a result ofwhich the distance may be less than satisfactory.

The value Hs−Hm obtained by subtracting the cross-sectional hardness(Hm) at an intermediate position between the core center and coresurface from the core surface hardness (Hs), although not particularlylimited, is preferably set to, in terms of Shore D hardness, from 11 to25. The upper limit in this value may be set to, in terms of Shore Dhardness, more preferably not more than 22, and even more preferably notmore than 20. The lower limit may be set to, in terms of Shore Dhardness, more preferably at least 12, and even more preferably at least15. If the above value is too large, the durability to cracking onrepeated impact may worsen. On the other hand, if this value is toosmall, the spin rate may rise excessively, as a result of which thedistance may be less than satisfactory.

The value Hs−Hc obtained by subtracting the core center hardness (Hc)from the core surface hardness (Hs), although not particularly limited,may be set to, in terms of Shore D hardness, preferably at least 16, andmore preferably at least 20. The upper limit in this value, although notparticularly limited, may be set to, in terms of Shore D hardness,preferably not more than 40, and more preferably not more than 30. Ifthis value is too small, the spin rate may rise excessively, as a resultof which the distance may be less than satisfactory.

In addition, the core center hardness (Hc), the core surface hardness(Hs) and the cross-sectional hardness (Hm) at an intermediate positionbetween the core center and the core surface, although not particularlylimited, preferably satisfy, in terms of Shore D hardnesses, thefollowing condition:

Hs−Hm>(Hm−Hc)×3.

In cases where the hardness relationship among the various parts of thecore departs from the above relationship, a good distance may not beachieved at both high head speeds and low head speeds.

The core deflection, i.e., the amount of deflection by the core whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf), while not subject to any particular limitation, may beset within a range of from 2.0 to 6.0 mm. In this case, the lower limitvalue is preferably at least 2.5 mm, more preferably at least 2.8 mm,and even more preferably at least 3.2 mm. The upper limit value may beset to preferably not more than 5.5 mm, more preferably not more than5.0 mm, and even more preferably not more than 4.5 mm. If the core istoo much harder than the above range (small deflection), the spin willrise excessively, which is unsuitable for the dimples of the presentinvention. On the other hand, if the core is too much softer than theabove range (large deflection), the feel of the ball at impact maybecome too soft and the durability to cracking on repeated impact mayworsen.

The specific gravity of the core, while not subject to any particularlimitation, may be set within a range of from 0.9 to 1.4. In such acase, the lower limit value is preferably at least 1.0, and morepreferably at least 1.1. The upper limit value may be set to preferablynot more than 1.3, and more preferably not more than 1.2.

In this invention, by using the above material to form the solid core 1,a golf ball capable of achieving a stable trajectory can be provided.

In the golf ball G of the invention, an inner cover layer 2 and an outercover layer 3 are formed over the above solid core 1. In this invention,the material hardnesses and thicknesses of each of these layers are setas described below. Here, “material hardness” refers to the hardness(Shore D) of a sheet of the cover material that has been molded underapplied pressure to a thickness of about 2 mm, as measured using a typeD durometer in general accordance with ASTM D2240.

First, the material hardness of the inner cover layer is set to, interms of Shore D hardness, at least 10, and may be set to preferably atleast 20, more preferably at least 30, and most preferably at least 40.The upper limit is set to, in terms of Shore D hardness, not more than60, and is recommended to be preferably not more than 57, morepreferably not more than 53, and most preferably not more than 50. Whenthe material hardness of the inner cover layer is too low, a sufficientrebound is not obtained, as a result of which, along with the reductionin the distance traveled by the ball when struck at a high HS, thedistance traveled by the ball when struck at a low HS also substantiallydecreases. On the other hand, if the material hardness is too high, thefeel of the ball at impact worsens.

The thickness of the inner cover layer is set to at least 0.8 mm, andmay be set to preferably at least 1.0 mm, more preferably at least 1.2mm, and even more preferably at least 1.5 mm. The upper limit is notmore than 3.0 mm, and is recommended to be preferably not more than 2.5mm, more preferably not more than 2.0 mm, and most preferably not morethan 1.6 mm. When the inner cover layer is too thin, the durability willworsen; when it is too thick, the ball will have a poor feel at impact.

The deflection by a sphere composed of the above core encased by theinner cover layer (inner cover layer-encased sphere), when compressedunder a final load of 1,275 N (130 kgf) from an initial load state of 98N (10 kgf), although not subject to any particular limitation, may beset in the range of 2.0 to 5.5 mm. In this case, the lower limit ispreferably at least 2.2 mm, more preferably at least 2.5 mm, and evenmore preferably at least 2.8 mm. The upper limit may be set topreferably not more than 5.0 mm, more preferably not more than 4.5 mm,and even more preferably not more than 4.0 mm. If the deflection is toosmall, the feel at impact may be too hard. On the other hand, if thedeflection is too large, the feel at impact may be too soft and thedurability to cracking may be poor.

The material hardness of the outer cover layer, in terms of Shore Dhardness, is set to at least 45, and may be set to preferably at least50, more preferably at least 52, even more preferably at least 54, andmost preferably at least 55. The upper limit is not more than 62, and isrecommended to be preferably not more than 61, and more preferably notmore than 60. If the material hardness of the outer cover layer is toolow, the feel at impact will be too soft or a sufficient rebound willnot be obtained, as a result of which, along with the reduction indistance traveled by the ball when hit at a high HS, the distancetraveled by the ball when hit at a low HS also substantially decreases.On the other hand, if the material hardness is too high, the durabilityworsens or the feel of the ball at impact worsens.

The thickness of the outer cover layer is set to at least 0.7 mm, andmay be set to preferably at least 1.0 mm, and more preferably at least1.2 mm. The upper limit is not more than 3.0 mm, preferably not morethan 2.5 mm, more preferably not more than 2.0 mm, and even morepreferably not more than 1.5 mm. If the outer cover layer is too thin, agood feel at impact is not obtained. On the other hand, if it is toothick, the durability worsens.

Moreover, in the present invention, the cover is formed so that thematerial hardness of the outer cover layer is higher than the materialhardness of the inner cover layer. In this case, the difference inhardness between the outer cover layer and the inner cover layer (outercover layer material hardness−inner cover layer material hardness),although not subject to any particular limitation, may be set so as tobe preferably at least 5, more preferably at least 6, and even morepreferably at least 7. It is recommended that the upper limit in thishardness difference be preferably not more than 50, more preferably notmore than 40, even more preferably not more than 30, and most preferablynot more than 20. If the hardness difference is too small, the feel atimpact may worsen; on the other hand, if it is too large, the durabilitymay worsen.

The ball having the above outer cover layer formed therein has adeflection when compressed under a final load of 1,275 N (130 kgf) froman initial load state of 98 N (10 kgf) which, although not particularlylimited, may be set in the range of 2.0 to 5.0 mm. The lower limit inthis case is preferably at least 2.2 mm, more preferably at least 2.3mm, and even more preferably at least 2.4 mm. The upper limit may be setto preferably not more than 4.5 mm, more preferably not more than 4.0mm, and even more preferably not more than 3.5 mm. If the deflection istoo small, the feel at impact may be too hard. On the other hand, if thedeflection is too large, the feel at impact may be too soft or thedurability to cracking may be poor.

Moreover, the ball has a deflection when compressed under a final loadof 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) which,although not particularly limited, may be set in the range of 7.0 to14.0 mm. The lower limit in this case is preferably at least 8.0 mm,more preferably at least 8.5 mm, and even more preferably at least 9.0mm. The upper limit may be set to preferably not more than 13.0 mm, morepreferably not more than 12.0 mm, and even more preferably not more than11.5 mm. If the deflection is too small, the feel at impact may be toohard. On the other hand, if the deflection is too large, the feel atimpact may be too soft or the durability to cracking may be poor.

The inner cover layer and the outer cover layer preferably satisfy thefollowing conditions in their relationship with the solid core.

The value obtained by subtracting the material hardness of the innercover layer from the surface hardness (Hs) of the core, which value isexpressed as (Hs−inner cover layer material hardness), although notparticularly limited, is preferably set to, in terms of Shore Dhardness, greater than −10 and less than +10. The upper limit, in termsof Shore D hardness, may be set to more preferably not more than +7, andeven more preferably not more than +4. The lower limit, in terms ofShore D hardness, may be set to more preferably at least −8, and evenmore preferably at least −5. If this value is too large, the ball mayhave an insufficient rebound or the spin rate of the ball may become toohigh. On the other hand, if this value is too small, the feel at impactmay harden or the durability to cracking on repeated impact may worsen.

The value obtained by subtracting the material hardness of the outercover layer from the surface hardness (Hs) of the core, which value isexpressed as (Hs−outer cover layer material hardness), although notparticularly limited, is preferably set to, in terms of Shore Dhardness, at least −15 and not higher than +5. The upper limit, in termsof Shore D hardness, may be set to more preferably not more than 2, andeven more preferably not more than 0. The lower limit, in terms of ShoreD hardness, may be set to more preferably at least −10, and even morepreferably at least −5. If this value is too large, the ball may have aninsufficient rebound or the spin rate of the ball may become too high.On the other hand, if this value is too small, the feel at impact maybecome hard or the durability to cracking on repeated impact may worsen.

In this invention, the ratio of deflection by a sphere composed of thesolid core encased by the inner cover layer (inner cover layer-encasedsphere) when compressed under a final load of 1,275 N (130 kgf) from aninitial load state of 98 N (10 kgf) to deflection by the solid core whencompressed under a final load of 1,275 N (130 kgf) from an initial loadstate of 98 N (10 kgf), which ratio is represented as (inner coverlayer-encased sphere deflection)/(solid core deflection), is preferablyfrom 0.82 to 0.92. The lower limit in this deflection ratio is morepreferably at least 0.84, and even more preferably at least 0.86. Theupper limit in this deflection ratio is more preferably not more than0.90, and even more preferably not more than 0.88. If this deflectionratio is too large, the ball rebound may be inadequate or the spin ratemay become too high. On the other hand, if this value is too small, thefeel at impact may harden or the durability to cracking on repeatedimpact may worsen.

In addition, although not particularly limited, the ratio of deflectionby the ball when compressed under a final load of 1,275 N (130 kgf) froman initial load state of 98 N (10 kgf) to deflection by the solid corewhen compressed under a final load of 1,275 N (130 kgf) from an initialload state of 98 N (10 kgf), which ratio is represented as (balldeflection)/(solid core deflection), is preferably from 0.72 to 0.79.The lower limit in this deflection ratio is more preferably at least0.74, and the upper limit is preferably not more than 0.77. If thisdeflection ratio is too large, the ball rebound may be inadequate or thespin rate may become too high. On the other hand, if this value is toosmall, the feel at impact may harden or the durability to cracking onrepeated impact may worsen.

Moreover, although not particularly limited, the ratio of deflection bythe ball when compressed under a final load of 5,880 N (600 kgf) from aninitial load state of 98 N (10 kgf) to deflection by the ball whencompressed under a final load of 1,275 N (130 kgf) from an initial loadstate of 98 N (10 kgf), which ratio is represented as (600 kgfdeflection/130 kgf deflection), is preferably from 3.2 to 3.7. The lowerlimit in this deflection ratio is more preferably at least 3.3, and theupper limit is preferably not more than 3.6. If this deflection ratio istoo large, the durability to cracking under repeated impact may worsen.On the other hand, if this value is too small, the spin rate may becomehigh and the distance traveled by the ball may decrease regardless ofthe head speed at which the ball is struck.

The cover having the above construction may be formed of a knownmaterial exemplified by thermoplastic resins such as ionomeric resins,and various types of thermoplastic elastomers. Examples of thermoplasticelastomers include polyester-based thermoplastic elastomers,polyamide-based thermoplastic elastomers, polyurethane-basedthermoplastic elastomers, olefin-based thermoplastic elastomers andstyrene-based thermoplastic elastomers.

In the present invention, such cover materials are not subject to anyparticular limitation, although preferred use may be made of a covermaterial composed primarily of a material selected from the groupconsisting of the polyurethane materials (I), polyurethane materials(II) and ionomeric resin materials shown below. These materials, andmolding methods for the same, are described in order below.

Polyurethane Material (I)

This material (I) is composed primarily of components A and B below:

(A) a thermoplastic polyurethane material,(B) an isocyanate mixture obtained by dispersing (B-1) an isocyanatecompound having as functional groups at least two isocyanate groups permolecule in (B-2) a thermoplastic resin that is substantiallynon-reactive with isocyanate.

Golf balls in which the cover has been formed of this material (I) canbe endowed with an excellent feel, controllability, cut resistance,scuff resistance and durability to cracking on repeated impact.

Next, each of the above components is described.

The thermoplastic polyurethane material (A) has a structure whichincludes soft segments made of a polymeric polyol (polymeric glycol),and hard segments made of a chain extender and a diisocyanate. Here, thepolymeric polyol used as a starting material is not subject to anyparticular limitation, and may be any that is used in the prior artrelating to thermoplastic polyurethane materials, such as polyesterpolyols and polyether polyols. Polyether polyols are preferable topolyester polyols because they enable the synthesis of thermoplasticpolyurethane materials having a high rebound resilience and excellentlow-temperature properties. Illustrative examples of polyether polyolsinclude polytetramethylene glycol and polypropylene glycol.Polytetramethylene glycol is especially preferred from the standpoint ofthe rebound resilience and low-temperature properties. The polymericpolyol has an average molecular weight of preferably from 1,000 to5,000. A molecular weight of from 2,000 to 4,000 is especially preferredfor synthesizing thermoplastic polyurethane materials having a highrebound resilience.

The chain extender employed is preferably one which is used in the artrelating to conventional thermoplastic polyurethane materials.Illustrative, non-limiting, examples include 1,4-butylene glycol,1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and2,2-dimethyl-1,3-propanediol. These chain extenders have an averagemolecular weight of preferably from 20 to 15,000.

The diisocyanate employed is preferably one which is used in the artrelating to conventional thermoplastic polyurethane materials.Illustrative, non-limiting, examples include aromatic diisocyanates suchas 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate and2,6-toluene diisocyanate; and aliphatic diisocyanates such ashexamethylene diisocyanate. However, depending on the type ofisocyanate, the crosslinking reaction during injection molding may bedifficult to control. In the practice of the invention, for stablereactivity with the subsequently described isocyanate mixture (B), it ismost preferable to use the following aromatic diisocyanate:4,4′-diphenylmethane diisocyanate.

A commercial product may be advantageously used as the thermoplasticpolyurethane material composed of the above-described material.Illustrative examples include those available under the trade namesPandex T-8290, Pandex T-8295 and Pandex T8260 (DIC Bayer Polymer, Ltd.),and those available under the trade names Resamine 2593 and Resamine2597 (Dainichi Seika Colour & Chemicals Mfg. Co., Ltd.).

The isocyanate mixture (B) is obtained by dispersing (B-1) an isocyanatecompound having as functional groups at least two isocyanate groups permolecule in (B-2) a thermoplastic resin that is substantiallynon-reactive with isocyanate. Here, the isocyanate compound (B-1) ispreferably an isocyanate compound used in the prior art relating tothermoplastic polyurethane materials. Illustrative, non-limiting,examples include aromatic diisocyanates such as 4,4′-diphenylmethanediisocyanate, 2,4-toluene diisocyanate and 2,6-toluene diisocyanate; andaliphatic diisocyanates such as hexamethylene diisocyanate. From thestandpoint of reactivity and work safety, the use of4,4′-diphenylmethane diisocyanate is most preferred.

The thermoplastic resin (B-2) is preferably a resin having a low waterabsorption and excellent compatibility with thermoplastic polyurethanematerials. Illustrative examples of such resins include polystyreneresins, polyvinyl chloride resins, ABS resins, polycarbonate resins, andpolyester elastomers (e.g., polyether-ester block copolymers,polyester-ester block copolymers). From the standpoint of the reboundresilience and strength, the use of a polyester elastomer, particularlya polyether-ester block copolymer, is especially preferred.

In the isocyanate mixture (B), it is desirable for the relativeproportions of the thermoplastic resin (B-2) and the isocyanate compound(B-1), expressed as the weight ratio (B-2):(B-1), to be from 100:5 to100:100, and especially from 100:10 to 100:40. If the amount of theisocyanate compound (B-1) relative to the amount of the thermoplasticresin (B-2) is too small, a greater amount of the isocyanate mixture (B)will have to be added in order to achieve an amount of additionsufficient for the crosslinking reaction with the thermoplasticpolyurethane material (A). As a result, the thermoplastic resin (B-2)will exert a large influence, rendering the physical properties of thematerial inadequate. On the other hand, if the amount of the isocyanatecompound (B-1) relative to the amount of the thermoplastic resin (B-2)is too large, the isocyanate compound (B-1) may cause slippage to occurduring mixing, making preparation of the isocyanate mixture (B)difficult.

The isocyanate mixture (B) may be obtained by, for example, adding theisocyanate compound (B-1) to the thermoplastic resin (B-2) andthoroughly working together these components at a temperature of from130 to 250° C. using mixing rolls or a Banbury mixer, then eitherpelletizing or cooling and subsequently grinding. A commercial productsuch as that available under the trade name Crossnate EM30 (DainichiSeika Colour & Chemicals Mfg. Co., Ltd.) may be suitably used as theisocyanate mixture (B).

The above material (I) is composed primarily of the thermoplasticpolyurethane material (A) and the isocyanate mixture (B) describedabove. In this material (I), the isocyanate mixture (B) is included inan amount, per 100 parts by weight of the thermoplastic polyurethanematerial (A), of at least 1 part by weight, preferably at least 5 partsby weight, and more preferably at least 10 parts by weight, but not morethan 100 parts by weight, preferably not more than 50 parts by weight,and more preferably not more than 30 parts by weight. If too littleisocyanate mixture (B) is included relative to the thermoplasticpolyurethane material (A), a sufficient crosslinking effect will not beachieved. On the other hand, if too much is included, this may lead todiscoloration of the molded material by unreacted isocyanate, which isundesirable.

In addition to above components (A) and (B), another component (C),although not essential, may also be included in the material (I). Thisother component is exemplified by thermoplastic polymeric materialsother than thermoplastic polyurethane materials; illustrative examplesinclude polyester elastomers, polyamide elastomers, ionomeric resins,styrene block elastomers, polyethylene, and nylon resins. When component(C) is included, the amount is not subject to any particular limitationand may be suitably selected as appropriate for such purposes asadjusting the hardness, improving the resilience, improving the flowproperties, and improving the adhesion of the cover material. The amountof component (C) included per 100 parts by weight of component (A) isset to preferably at least 10 parts by weight, and the upper limit isset to not more than 100 parts by weight, preferably not more than 75parts by weight, and more preferably not more than 50 parts by weight.If necessary, various additives such as pigments, dispersants,antioxidants, light stabilizers, ultraviolet absorbers and partingagents may also be suitably included in the above material (I).

Formation of the cover using the above material (I) may be carried outby a known molding method. For example, the cover may be molded byadding the isocyanate mixture (B) to the thermoplastic polyurethanematerial (A) and dry mixing, feeding the resulting mixture to aninjection molding machine, and injecting the molten resin blend over thecore. In such a case, the molding temperature varies with the type ofthermoplastic polyurethane material (A), although molding is generallycarried out within the temperature range of 150 to 250° C.

Reactions and crosslinking which take place in the golf ball coverobtained as described above are believed to involve the reaction ofisocyanate groups with hydroxyl groups remaining in the thermoplasticpolyurethane material to form urethane bonds, or the creation of anallophanate or biuret crosslinked form via a reaction involving theaddition of isocyanate groups to urethane groups in the thermoplasticpolyurethane material. Although the crosslinking reactions have not yetproceeded to a sufficient degree immediately after injection molding ofthe material (I), the crosslinking reactions can be made to proceedfurther by carrying out an annealing step after molding, in this waymaintaining characteristics which are useful for a golf ball cover.“Annealing,” as used herein, refers to heat aging the cover at aconstant temperature for a fixed length of time, or aging the cover fora fixed period at room temperature.

Polyurethane Material (II)

This material (II) is a single resin blend in which the primarycomponents are (D) a thermoplastic polyurethane and (E) a polyisocyanatecompound. By forming a cover composed primarily of such a polyurethanematerial (II), it is possible to achieve an excellent feel,controllability, cut resistance, scuff resistance and durability tocracking on repeated impact without a loss of resilience.

As used herein, reference to a “single” resin blend means that the resinblend is not fed as a plurality of types of pellets, but rather issupplied to, for example, an injection molding machine as one type ofpellet prepared by incorporating a plurality of ingredients intoindividual pellets.

To fully and effectively achieve the objects of the invention, anecessary and sufficient amount of unreacted isocyanate groups should bepresent within the cover resin material. Specifically, it is recommendedthat the combined weight of above components (D) and (E) account for atleast 60%, and more preferably at least 70%, of the total weight of thecover. Components (D) and (E) are described in detail below.

The above thermoplastic polyurethane (D) is described. The thermoplasticpolyurethane structure includes soft segments made of a polymeric polyol(polymeric glycol) that is a long-chain polyol, and hard segments madeof a chain extender and a polyisocyanate compound. Here, the long-chainpolyol used as a starting material is not subject to any particularlimitation, and may be any that has hitherto been used in the artrelating to thermoplastic polyurethanes. Exemplary long-chain polyolsinclude polyester polyols, polyether polyols, polycarbonate polyols,polyester polycarbonate polyols, polyolefin polyols, conjugated dienepolymer-based polyols, castor oil-based polyols, silicone-based polyolsand vinyl polymer-based polyols. These long-chain polyols may be usedsingly or as combinations of two or more thereof. Of the long-chainpolyols mentioned here, polyether polyols are preferred because theyenable the synthesis of thermoplastic polyurethanes having a highrebound resilience and excellent low-temperature properties.

Illustrative examples of the above polyether polyol includepoly(ethylene glycol), poly(propylene glycol), poly(tetramethyleneglycol) and poly(methyltetramethylene glycol) obtained by thering-opening polymerization of cyclic ethers. These polyether polyolsmay be used singly or as a combination of two or more thereof. In thepresent invention, poly(tetramethylene glycol) andpoly(methyltetramethylene glycol) are preferred.

It is preferable for these long-chain polyols to have a number-averagemolecular weight in the range of 1,500 to 5,000. By using a long-chainpolyol having a number-average molecular weight within this range, golfballs made with a thermoplastic polyurethane composition havingexcellent properties such as resilience and manufacturability can bereliably obtained. The number-average molecular weight of the long-chainpolyol is more preferably in the range of 1,700 to 4,000, and even morepreferably in the range of 1,900 to 3,000.

As used herein, “number-average molecular weight of the long-chainpolyol” refers to the number-average molecular weight calculated basedon the hydroxyl number measured in accordance with JIS K-1557.

Any chain extender employed in the prior art relating to thermoplasticpolyurethane materials may be advantageously used as the chain extender.For example, low-molecular-weight compounds with a molecular weight of400 or less which have on the molecule two or more active hydrogen atomscapable of reacting with isocyanate groups are preferred. Illustrative,non-limiting, examples of the chain extender include 1,4-butyleneglycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and2,2-dimethyl-1,3-propanediol. In the present invention, an aliphaticdiol having 2 to 12 carbons is preferred, and 1,4-butylene glycol ismore preferred.

Any polyisocyanate compound employed in the prior art relating tothermoplastic polyurethane materials may be advantageously used withoutparticular limitation as the polyisocyanate compound. For example, usemay be made of one or more selected from the group consisting of4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, p-phenylene diisocyanate, xylylene diisocyanate,1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate,hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate,tetramethylene diisocyanate, hexamethylene diisocyanate, isophoronediisocyanate, norbornene diisocyanate, trimethylhexamethylenediisocyanate and dimer acid diisocyanate. However, depending on the typeof isocyanate, the crosslinking reaction during injection molding may bedifficult to control. In the practice of the invention, to provide abalance between stability at the time of production and the propertiesthat are manifested, it is most preferable to use 4,4′-diphenylmethanediisocyanate, which is an aromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving asabove component D to be a thermoplastic polyurethane synthesized using apolyether polyol as the long-chain polyol, using an aliphatic diol asthe chain extender, and using an aromatic diisocyanate as thepolyisocyanate compound. It is desirable, though not essential, for thepolyether polyol to be a polytetramethylene glycol having anumber-average molecular weight of at least 1,900, for the chainextender to be 1,4-butylene glycol, and for the aromatic diisocyanate tobe 4,4′-diphenylmethane diisocyanate.

The mixing ratio of active hydrogen atoms to isocyanate groups in theabove polyurethane-forming reaction can be adjusted within a desirablerange so as to make it possible to obtain a golf ball which is composedof a thermoplastic polyurethane composition and has various improvedproperties, such as rebound, spin performance, scuff resistance andmanufacturability. Specifically, in preparing a thermoplasticpolyurethane by reacting the above long-chain polyol, polyisocyanatecompound and chain extender, it is desirable to use the respectivecomponents in proportions such that the amount of isocyanate groups onthe polyisocyanate compound per mole of active hydrogen atoms on thelong-chain polyol and the chain extender is from 0.95 to 1.05 moles.

No particular limitation is imposed on the method of preparing component(D). Production may be carried out by either a prepolymer process or aone-shot process in which the long-chain polyol, chain extender andpolyisocyanate compound are used and a known urethane-forming reactionis effected. Of these, a process in which melt polymerization is carriedout in a substantially solvent-free state is preferred. Production bycontinuous melt polymerization using a multiple screw extruder isespecially preferred.

A commercial product may be used as component (D). Illustrative examplesinclude products available under the trade names Pandex T8295, PandexT8290 and Pandex T8260 (DIC Bayer Polymer, Ltd.).

Next, concerning the polyisocyanate compound used as component E, it isessential that, in at least some portion thereof within a single resinblend, all the isocyanate groups on the molecule remain in an unreactedstate. That is, polyisocyanate compound in which all the isocyanategroups on the molecule are in a completely free state should be presentwithin a single resin blend, and such a polyisocyanate compound may bepresent together with a polyisocyanate compound in which a portion ofthe isocyanate groups on the molecule are in a free state.

Various isocyanates may be used without particular limitation as thepolyisocyanate compound. Specific examples include one or more selectedfrom the group consisting of 4,4′-diphenylmethane diisocyanate,2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylenediisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate,tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate,dicyclohexylmethane diisocyanate, tetramethylene diisocyanate,hexamethylene diisocyanate, isophorone diisocyanate, norbornenediisocyanate, trimethylhexamethylene diisocyanate and dimer aciddiisocyanate. Of the above group of isocyanates, using4,4′-diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate andisophorone diisocyanate is preferred for achieving a good balancebetween the influence on moldability by, for example, the rise inviscosity associated with reaction with the thermoplastic polyurethaneserving as component D, and the properties of the resulting golf ballcover material.

In Polyurethane Material (II), although not an essential ingredient, athermoplastic elastomer other than the above thermoplastic polyurethanemay be included as component F in addition to above components D and E.Including this component F in the above resin blend enables the flowproperties of the resin blend to be further improved and enables variousproperties required of golf ball cover materials, such as resilience andscuff resistance, to be enhanced.

This component F, which is a thermoplastic elastomer other than theabove thermoplastic polyurethane, is exemplified by one or morethermoplastic elastomer selected from among polyester elastomers,polyamide elastomers, ionomeric resins, styrene block elastomers,hydrogenated styrene-butadiene rubbers,styrene-ethylene/butylene-ethylene block copolymers and modified formsthereof, ethylene-ethylene/butylene-ethylene block copolymers andmodified forms thereof, styrene-ethylene/butylene-styrene blockcopolymers and modified forms thereof, ABS resins, polyacetals,polyethylenes and nylon resins. The use of polyester elastomers,polyamide elastomers and polyacetals is especially preferred because theresilience and scuff resistance are enhanced, owing to reactions withisocyanate groups, while at the same time a good manufacturability isretained.

The relative proportions of above components D, E and F are not subjectto any particular limitation. However, to fully achieve the advantageouseffects of the invention, it is preferable for the weight ratio amongthe respective components to be (D):(E):(F)=100:2 to 50:0 to 50, andmore preferably (D):(E):(F)=100:2 to 30:8 to 50.

In this invention, a single resin blend for forming the cover isprepared by mixing together component D, component E, and also optionalcomponent F. At this time, it is essential to select the mixingconditions such that, of the polyisocyanate compound, at least somepolyisocyanate compound is present in which all the isocyanate groups onthe molecule remain in an unreacted state. For example, treatment suchas mixture in an inert gas (e.g., nitrogen) or in a vacuum state must befurnished. The resin blend is then injection-molded around a core whichhas been placed in a mold. To smoothly and easily handle the resinblend, it is preferable for the blend to be formed into pellets having alength of 1 to 10 mm and a diameter of 0.5 to 5 mm. Sufficientisocyanate groups in an unreacted state remain in these resin pellets;the unreacted isocyanate groups react with component D or component F toform a crosslinked material while the resin blend is beinginjection-molded about the core, or due to post-treatment such asannealing thereafter.

In addition, various optional additives may also be included in thiscover-forming resin blend. For example, pigments, dispersants,antioxidants, light stabilizers, ultraviolet absorbers, and partingagents may be suitably included.

The melt mass flow rate (MFR) of this resin blend at 210° C. is notsubject to any particular limitation. However, to increase the flowproperties and manufacturability, the MFR is preferably at least 5 g/10min, and more preferably at least 6 g/10 min. If the melt mass flow rateof the resin blend is too low, the flow properties will decrease, whichmay cause eccentricity during injection molding and may also lower thedegree of freedom in the thickness of the cover that can be molded. Themelt mass flow rate is a measured value obtained in accordance withJIS-K7210 (1999 edition).

The method of molding the cover may involve feeding the above resinblend to an injection-molding machine and injecting the molten resinblend around the core. Although the molding temperature in this casewill vary depending on the type of thermoplastic polyurethane, themolding temperature is generally from 150 to 250° C.

When injection molding is carried out, it is desirable, though notessential, to carry out such molding in a low-humidity environment bysubjecting some or all places on the resin paths from the resin feedarea to the mold interior to purging with an inert gas such as nitrogenor a low-moisture gas such as low dew-point dry air, or to vacuumtreatment. Preferred, non-limiting, examples of the medium used fortransporting the resin under applied pressure include low-moisture gasessuch as low dew-point dry air or nitrogen gas. By carrying out moldingin such a low-humidity environment, the progression of reactions byisocyanate groups before the resin blend is charged into the moldinterior is suppressed. By thus including, within the molded resinmaterial, polyisocyanate in which some isocyanate groups are present inan unreacted state, it is possible to reduce variable factors such as anundesirable rise in viscosity and to increase the real crosslinkingefficiency.

Techniques that may be used to confirm the presence of polyisocyanatecompound in an unreacted state within the resin blend prior to injectionmolding about the core include those which involve extraction with asuitable solvent that selectively dissolves out only the polyisocyanatecompound. An example of a simple and convenient method is one in whichconfirmation is carried out by simultaneous thermogravimetric anddifferential thermal analysis (TG-DTA) measurement in an inertatmosphere. For example, when the above-described single resin blend(Polyurethane Material (II)) is heated in a nitrogen atmosphere at atemperature ramp-up rate of 10° C./min, a gradual drop in the weight ofdiphenylmethane diisocyanate can be observed from about 150° C. On theother hand, in a resin sample in which the reaction between thethermoplastic polyurethane material and the isocyanate mixture has beencarried out to completion, a weight drop is not observed from about 150°C., but a weight drop can be observed from about 230 to 240° C.

After the above Polyurethane Material (II) has been injection-molded toform a cover, the properties as a golf ball cover can be additionallyimproved by carrying out annealing so as to induce the crosslinkingreaction to proceed further. “Annealing,” as used herein, refers toaging the cover in a fixed environment for a fixed length of time.

Ionomeric Resin Material

In the present invention, “ionomeric resin material” refers to a resincomposition which includes: 100 parts by weight of a resin componentcomposed of a base resin containing (a) from 95 to 50 wt % of anolefin-unsaturated carboxylic acid-unsaturated carboxylic acid esterrandom copolymer and/or a metal salt thereof and (b) from 0 to 20 wt %of an olefin-unsaturated carboxylic acid random copolymer and/or a metalsalt thereof, and

(c) from 0 to 50 wt % of a thermoplastic block copolymer composed of acrystalline polyolefin block and a polyethylene/butylene randomcopolymer;

(d) from 5 to 170 parts by weight of a fatty acid or fatty acidderivative having a molecular weight of 280 to 1,500; and(e) from 0.1 to 10 parts by weight of a basic inorganic metal compoundcapable of neutralizing acid groups in components (a) and (d), and, ifnecessary, component (b).

Components (a) to (e) are described below.

Component (a) and component (b) serve as the base resin of the aboveresin composition. Component (a) is an olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester random copolymer and/or a metalsalt thereof, and component (b) is an olefin-unsaturated carboxylic acidrandom copolymer and/or a metal salt thereof. In the present invention,either of above components (a) and (b) may be used singly or both mayused in combination.

Here, above component (a) has a weight-average molecular weight (Mw) ofpreferably at least 100,000, more preferably at least 110,000, and evenmore preferably at least 120,000, but preferably not more than 200,000,more preferably not more than 190,000, and even more preferably not morethan 170,000. The weight-average molecular weight (Mw) to number-averagemolecular weight (Mn) ratio for the copolymer is preferably at least 3,and more preferably at least 4, with the upper limit being preferablynot more than 7, and more preferably not more than 6.5.

The olefin in component (a) generally has a number of carbons that is atleast 2, but not more than 8, and preferably not more than 6.Illustrative examples of such olefins include ethylene, propylene,butene, pentene, hexene, heptene and octene. Ethylene is especiallypreferred.

Illustrative examples of the unsaturated carboxylic acid include acrylicacid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid andmethacrylic acid are especially preferred.

The unsaturated carboxylic acid ester may be, for example, a lower alkylester of an unsaturated carboxylic acid. Illustrative examples includemethyl methacrylate, ethyl methacrylate, propyl methacrylate, butylmethacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butylacrylate. The use of butyl acrylate (n-butyl acrylate, isobutylacrylate) is especially preferred.

The random copolymer serving as component (a) may be obtained by therandom copolymerization of the above ingredients in accordance with aknown method. Here, the unsaturated carboxylic acid content (acidcontent) within the random copolymer, although not subject to anyparticular limitation, may be set to generally at least 2 wt %,preferably at least 6 wt %, and more preferably at least 8 wt %. It isrecommended that the upper limit in the unsaturated carboxylic acidcontent (acid content), although not subject to any particularlimitation, be generally not more than 25 wt %, preferably not more than20 wt %, and more preferably not more than 15 wt %. At a low acidcontent, the rebound may decrease, whereas at a high acid content, theprocessability of the material may decrease.

The copolymer of component (a) accounts for a proportion of the overallbase resin which is preferably from 95 to 50 wt %. The lower limit ofthis proportion is preferably at least 60 wt %, more preferably at least70 wt %, and even more preferably at least 75 wt %. The upper limit ispreferably not more than 92 wt %, more preferably not more than 89 wt %,and most preferably not more than 86 wt %.

The metal salt of the copolymer of component (a) may be obtained byneutralizing some of the acid groups in the random copolymer ofcomponent (a) with metal ions. Here, the metal ions which neutralize theacid groups are exemplified by Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺,Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. In the present invention, of these, preferred usemay be of Na⁺, Li⁺, Zn⁺⁺, Mg⁺⁺ and Ca⁺⁺ in particular, and Zn⁺⁺ is evenmore recommended. The degree of neutralization of the random copolymerby these metal ions, while not subject to any particular limitation, isgenerally at least 5 mol %, preferably at least 10 mol %, and especiallyat least 20 mol %. It is recommended that the upper limit in the degreeof neutralization, while not subject to any particular limitation, begenerally not more than 95 mol %, preferably not more than 90 mol %, andespecially not more than 80 mol %. At a degree of neutralization inexcess of 95 mol %, the moldability may decrease. On the other hand, atless than 5 mol %, it is necessary to increase the amount in which theinorganic metal compound serving as component (c) is added, which maypresent a drawback in terms of cost. Such a neutralization product maybe obtained by a known method. For example, the neutralization productmay be obtained by introducing a metal ion compound, such as a formate,acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide,into the random copolymer.

A commercial product may be used as component (a). Illustrative examplesof olefin-unsaturated carboxylic acid-unsaturated carboxylic acid esterrandom copolymers include those available under the trade names NucrelAN4318, Nucrel AN4319, and Nucrel AN4311 (DuPont-Mitsui PolychemicalsCo., Ltd.). Illustrative examples of metal salts of olefin-unsaturatedcarboxylic acid-unsaturated carboxylic acid ester random copolymersinclude those available under the trade names Himilan AM7316, HimilanAM7331, Himilan 1855 and Himilan 1856 (DuPont-Mitsui Polychemicals Co.,Ltd.), and those available under the trade names Surlyn 6320 and Surlyn8120 (E.I. DuPont de Nemours and Co., Ltd.).

Next, it is recommended that the weight-average molecular weight (Mw) ofcomponent (b) be preferably at least 100,000, more preferably at least110,000, and even more preferably at least 120,000, and that the upperlimit thereof be preferably not more than 200,000, more preferably notmore than 190,000, and even more preferably not more than 170,000. Theweight-average molecular weight (Mw) to number-average molecular weight(Mn) ratio for the copolymer is preferably at least 3, and morepreferably at least 4, and the upper limit thereof is preferably notmore than 7, and more preferably not more than 6.5.

Here, the olefin in component (b) is generally an olefin in which thenumber of carbons is at least 2 but not more than 8, and preferably notmore than 6. Illustrative examples include ethylene, propylene, butene,pentene, hexene, heptene and octene. The use of ethylene is especiallypreferred.

Illustrative examples of the unsaturated carboxylic acid in component(b) include acrylic acid, methacrylic acid, maleic acid and fumaricacid. Acrylic acid and methacrylic acid are especially preferred.

In addition, the random copolymer serving as component (b) may beobtained by the random copolymerization of the above ingredients inaccordance with a known method. Here, the unsaturated carboxylic acidcontent (acid content) within the random copolymer, while not subject toany particular limitation, may be set to generally at least 2 wt %,preferably at least 6 wt %, and more preferably at least 8 wt %. Noparticular limitation is imposed on the upper limit in the unsaturatedcarboxylic acid content (acid content), although it is recommended thatthis be generally not more than 25 wt %, preferably not more than 20 wt%, and more preferably not more than 15 wt %. At a low acid content,there is a possibility that the rebound will decrease, whereas at a highacid content, there is a possibility that the material processabilitywill decrease.

In the above case, the copolymer of component (b) accounts for aproportion of the overall base resin which may be set to more than 0,and may be set to preferably at least 1 wt %. The upper limit, althoughnot subject to any particular limitation, may be set to not more than 20wt %, preferably not more than 17 wt %, more preferably not more than 10wt %, even more preferably not more than 8 wt %, and most preferably notmore than 5 wt %.

The metal salt of the copolymer of component (b) may be obtained byneutralizing some of the acid groups in the random copolymer ofcomponent (b) with metal ions. Here, preferred use may be made of, forexample, Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ or Pb⁺⁺, asthe metal ions which neutralize the acid groups. In the presentinvention, of these, more preferred use may be made of Na⁺, Li⁺, Zn⁺⁺,Mg⁺⁺ or Ca⁺⁺. The use of Zn⁺⁺ is especially recommended. The degree ofneutralization of the random copolymer by these metal ions, while notsubject to any particular limitation, may be set to generally at least 5mol %, preferably at least 10 mol %, and especially at least 20 mol %.It is recommended that the upper limit in the degree of neutralization,while not subject to any particular limitation, be set to generally notmore than 95 mol %, preferably not more than 90 mol %, and especiallynot more than 80 mol %. At a degree of neutralization in excess of 95mol %, the moldability may decrease. On the other hand, at less than 5mol %, there arises a need to increase the amount in which the inorganicmetal compound serving as component (c) is added, which may present adrawback in terms of cost. Such a neutralization product may be obtainedby a known method. For example, the neutralization product may beobtained by introducing a metal ion compound, such as a formate,acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide,into the random copolymer.

A commercial product may be used as component (b). Illustrative examplesinclude those available under the trade names Nucrel 1560, Nucrel 1525and Nucrel 1035 (DuPont-Mitsui Polychemicals Co., Ltd.). Illustrativeexamples of metal salts of the olefin-unsaturated carboxylic acid randomcopolymer include those available under the trade names Himilan 1605,Himilan 1601, Himilan 1557, Himilan 1705 and Himilan 1706 (DuPont-MitsuiPolychemicals Co., Ltd.), those available under the trade names Surlyn7930 and Surlyn 7920 (E.I. DuPont de Nemours and Co., Ltd.), and thoseavailable under the trade names Escor 5100 and Escor 5200 (ExxonMobilChemical).

Component (c) is a thermoplastic block copolymer composed of acrystalline polyolefin block and a polyethylene/butylene randomcopolymer. This component (c) is exemplified by thermoplastic blockcopolymers composed of a crystalline polyethylene block (E) as a hardsegment and a block of a relatively random copolymer of ethylene andbutylene (EB) as a soft segment. Preferred use may be made of blockcopolymers having a molecular structure with a hard segment at one orboth ends, such as block copolymers having an E-EB or E-EB-E structure.

Such a component (c) may be obtained by hydrogenating a polybutadiene.Here, the polybutadiene used in hydrogenation is preferably one in whichbonding within the butadiene structure is characterized by a 1,4-bondcontent in the butadiene structure as a whole of from 95 to 100 wt %,and in which from 50 to 100 wt %, and preferably from 80 to 100 wt %, ofthe 1,4-bonds are present as block-like regions.

The above-mentioned E-EB-E type thermoplastic block copolymer ispreferably one obtained by hydrogenating a polybutadiene having at bothends of the molecular chain 1,4-polymerization products which are richin 1,4-bonds and having an intermediate region where 1,4-bonds and1,2-bonds are intermingled. The degree of hydrogenation (conversion ofdouble bonds on the polybutadiene to saturated bonds) in thepolybutadiene hydrogenate is preferably from 60 to 100%, and morepreferably from 90 to 100%. Too low a degree of hydrogenation may giverise to undesirable effects such as gelation in the blending step withother components such as an ionomeric resin and, when the golf ball hasbeen formed, may lead to a poor durability to impact.

In the block copolymer having an E-EB or E-EB-E molecular structure witha hard segment at one or both ends that may be advantageously used asthe thermoplastic block copolymer, the content of the hard segments ispreferably from 10 to 50 wt %. If the hard segment content is too high,the cover may lack sufficient softness, making it difficult toeffectively achieve the objects of the invention. On the other hand, ifthe hard segment content is too low, the blend may have a poormoldability.

The thermoplastic block copolymer has a melt mass flow rate, at a testtemperature of 230° C. and a test load of 21.2 N, of preferably from0.01 to 15 g/10 min, and more preferably from 0.03 to 10 g/10 min.Outside of this range, problems such as weld lines, sink marks and shortshots may arise during injection molding. Moreover, it is preferable forthe thermoplastic block copolymer to have a surface hardness of from 10to 50. If the surface hardness is too low, the golf ball may have adecreased durability to repeated impact. On the other hand, if thesurface hardness is too high, a blend of the thermoplastic blockcopolymer with an ionomeric resin may have a decreased resilience. Thethermoplastic block copolymer has a number-average molecular weight ofpreferably from 30,000 to 800,000.

A commercial product may be used as component (c). Illustrative examplesinclude those available under the trade names Dynaron 6100P, Dynaron6200P and Dynaron 6201B (JSR Corporation). Of these, Dynaron 6100P,which is a block polymer having crystalline olefin blocks at both ends,is especially preferred for use in the present invention. These olefinicthermoplastic elastomers may be used singly or as mixtures of two ormore thereof.

In cases where component (c) is included in the resin component, theproportion of the total resin components accounted for by component (c)may be set to more than 0, and is preferably set to at least 5 wt %,more preferably at least 8 wt %, even more preferably at least 11 wt %,and most preferably at least 14 wt %. The upper limit, while not subjectto any particular limitation, may be set to preferably not more than 50wt %, more preferably not more than 40 wt %, even more preferably notmore than 30 wt %, and most preferably not more than 20 wt %.

Component (d) is a fatty acid or fatty acid derivative having amolecular weight of at least 280 but not more than 1,500 whose purposeis to enhance the flow properties of the resin composition. It has amolecular weight which is very small compared with those of components(a) to (c), and helps to significantly decrease the melt viscosity ofthe mixture. Also, because the fatty acid (or fatty acid derivative) ofcomponent (d) has a molecular weight of at least 280 but not more than1,500 and has a high content of acid groups (or derivative moietiesthereof), its addition results in little loss of rebound.

The fatty acid or fatty acid derivative serving as component (d) may bean unsaturated fatty acid or fatty acid derivative having a double bondor triple bond in the alkyl moiety, or it may be a saturated fatty acidor fatty acid derivative in which all the bonds in the alkyl moiety aresingle bonds. It is recommended that the number of carbon atoms on themolecule be generally at least 18, with an upper limit of not more than80, and especially not more than 40. Too few carbons may make itimpossible to achieve an improved heat resistance and may also set theacid group content so high as to cause the acid groups to interact withacid groups present in the base resin, diminishing the flow-improvingeffects. On the other hand, too many carbons increases the molecularweight, as a result of which significant flow-improving effects may notappear, which may make the material difficult to use.

Specific examples of fatty acids that may be used as component (d)include stearic acid, 12-hydroxystearic acid, behenic acid, oleic acid,linoleic acid, linolenic acid, arachidic acid and lignoceric acid. Ofthese, preferred use may be made of stearic acid, arachidic acid,behenic acid, lignoceric acid and oleic acid.

Fatty acid derivatives are exemplified by derivatives in which theproton on the acid group of the fatty acid has been substituted.Exemplary fatty acid derivatives of this type include metallic soaps inwhich the proton has been substituted with a metal ion. Metal ions thatmay be used in such metallic soaps include Li⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Mn⁺⁺,Al⁺⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Of these, Ca⁺⁺, Mg⁺⁺ andZn⁺⁺ are especially preferred.

Specific examples of fatty acid derivatives that may be used ascomponent (d) include magnesium stearate, calcium stearate, zincstearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc12-hydroxystearate, magnesium arachidate, calcium arachidate, zincarachidate, magnesium behenate, calcium behenate, zinc behenate,magnesium lignocerate, calcium lignocerate and zinc lignocerate. Ofthese, magnesium stearate, calcium stearate, zinc stearate, magnesiumarachidate, calcium arachidate, zinc arachidate, magnesium behenate,calcium behenate, zinc behenate, magnesium lignocerate, calciumlignocerate and zinc lignocerate are preferred.

The amount of component (d) included per 100 parts by weight of theresin component is at least 5 parts by weight, preferably at least 20parts by weight, more preferably at least 50 parts by weight, and evenmore preferably at least 85 parts by weight. The upper limit in theamount included per 100 parts by weight of the resin component is notmore than 170 parts by weight, preferably not more than 150 parts byweight, more preferably not more than 130 parts by weight, and even morepreferably not more than 110 parts by weight.

Use may also be made of known metallic soap-modified ionomers (see, forexample, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 andInternational Disclosure WO 98/46671) when using above components (a)and (b).

The basic inorganic metal compound of component (e) is included so as toneutralize acid groups in above component (a), component (d) and, ifnecessary, component (b). When above component (d) is not included, andin particular when a metal-modified ionomeric resin alone (e.g., a metalsoap-modified ionomeric resin of the type mentioned in the foregoingpatent publications, alone) is heated and mixed, as mentioned below, themetallic soap and unneutralized acid groups present on the ionomerundergo exchange reactions, generating a fatty acid. Because this fattyacid has a low thermal stability and readily vaporizes during molding,it causes molding defects. Moreover, if the fatty acid thus generateddeposits on the surface of the molded material, it substantially lowerspaint film adhesion.

To resolve such problems, a basic inorganic metal compound whichneutralizes the acid groups present in above components (a), (b) and (d)is thus included as an essential component (component (e)). By addingcomponent (e), the acid groups in above components (a), (b) and (d) areneutralized. Synergistic effects from the inclusion of these respectivecomponents increase the thermal stability of the resin composition whileat the same time conferring a good moldability, and also impart theexcellent property of enhancing rebound as a golf ball material.

It is recommended that component (e) be a basic inorganic metalcompound—preferably a monoxide or hydroxide—which is capable ofneutralizing acid groups in above components (a), (b) and (d). Becausesuch compounds have a high reactivity with the ionomeric resin and thereaction by-products contain no organic matter, the degree ofneutralization of the resin composition can be increased without a lossof thermal stability.

The metal ions used here in the basic inorganic metal compound areexemplified by Li⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Al⁺⁺⁺, Ni⁺, Fe⁺⁺, Fe⁺⁺⁺,Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Illustrative examples of the inorganicmetal compound include basic inorganic fillers containing these metalions, such as magnesium oxide, magnesium hydroxide, magnesium carbonate,zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calciumhydroxide, lithium hydroxide and lithium carbonate. Of these, as notedabove, a monoxide or hydroxide is preferred. The use of magnesium oxideor calcium hydroxide, which have high reactivities with ionomer resins,is especially preferred in the present invention.

Component (e) is included in an amount, per 100 parts by weight of theresin component, of from 0.1 to 10 parts by weight. In this case, thelower limit is preferably at least 0.5 part by weight, more preferablyat least 0.8 part by weight, and even more preferably at least 1 part byweight. The upper limit in the amount included per 100 parts by weightof the resin component is not more than 8 parts by weight, preferablynot more than 5 parts by weight, and more preferably not more than 4parts by weight.

The above-described resin composition which is obtained by blendingcomponents (a) to (e) can be provided with improved thermal stability,moldability and resilience. To this end, it is recommended that at least70 mol %, preferably at least 80 mol %, and more preferably at least 90mol %, of the acid groups in the resin composition be neutralized. Ahigh degree of neutralization more reliably suppresses the exchangereactions that pose a problem in the above-described cases wherecomponents (a) and (b) and the fatty acid (or fatty acid derivative)alone are used, thus making it possible to prevent the generation offatty acids. As a result, a material can be obtained which has amarkedly increased thermal stability, a good moldability, and asubstantially higher resilience than conventional ionomeric resins.

Here, with regard to neutralization of the above resin composition, tomore reliably achieve both a high degree of neutralization and good flowproperties, it is recommended that the acid groups in the resincomposition be neutralized with transition metal ions and with alkalimetal and/or alkaline earth metal ions. Because transition metal ionshave a weaker ionic cohesion than alkali metal and alkaline earth metalions, it is possible in this way to neutralize some of the acid groupsin the resin composition and thus enable the flow properties to besignificantly improved.

Various additives may also be optionally included in the above resincomposition. Examples of additives which may be suitably included arepigments, dispersants, antioxidants, ultraviolet absorbers and opticalstabilizers. Moreover, to further improve the feel of the golf ball onimpact, the resin composition may also include various non-ionomericthermoplastic elastomers. Illustrative examples of such non-ionomericthermoplastic elastomers include styrene-based thermoplastic elastomers,ester-based thermoplastic elastomers and urethane-based thermoplasticelastomers. In this invention, the use of styrene-based thermoplasticelastomers is especially preferred.

Use may be made of a known mixing apparatus when preparing the aboveresin composition. For example, the above respective ingredients may bemixed using a twin-screw extruder, a Banbury mixer or a kneader. In sucha case, the heating and mixing conditions may be suitably selectedaccording to the type of material, and are not subject to any particularlimitation. For example, mixing may be carried out at a temperature offrom 150 to 250° C. The method of molding the cover using the aboveresin composition is also not subject to any particular limitation. Forexample, use may be made of an injection molding method or a compressionmolding method. When injection molding is employed, the process mayinvolve placing a prefabricated core at a given position in theinjection mold, then introducing the above material into the mold. Whencompression molding is employed, the process may involve producing apair of half cups from the above material, covering the core with thesehalf-cups, then applying pressure and heat within a mold. If moldingunder heat and pressure is carried out, the molding conditions used maybe a temperature of from 120 to 170° C. and a period of from 1 to 5minutes.

The cover material used in the invention may be a known cover material.Although not subject to any particular limitation, preferred use may bemade of the above-described Polyurethane Material (I), PolyurethaneMaterial (II) or an ionomeric resin material.

In the inventive golf ball, by combining dimples which satisfy thesubsequently described specific parameters and are able to achieve arelatively low trajectory with an inner cover layer and an outer coverlayer having the specific constructions described above, it is possibleto greatly reduce the distance traveled by the golf ball on shots takenat a high head speed and also hold down the decrease in distancetraveled on shots taken at a low head speed. The parameters for thedimples formed in the inventive golf ball are described in detail below.

In the present invention, dimples having the following parameters (1) to(10) are formed on the surface of the cover made of the above-describedmaterial. In cases where the surface of the ball is subjected tofinishing treatment (e.g., painting and stamping) after the cover hasbeen formed, parameters (1) to (10) below are calculated based on theshape of the dimples on the finished ball in which such treatment hasbeen fully completed.

Dimple Parameter (1)

The total number of dimples is set in a range of at least 250 but notmore than 500. The lower limit in the number of dimples may be set topreferably at least 280, more preferably at least 300, and even morepreferably at least 340. The upper limit may be set to preferably notmore than 450, more preferably not more than 420, and even morepreferably not more than 400. In this range, the golf ball readilyincurs lift, enabling the ball to travel farther, particularly on shotswith a driver.

Dimple Parameter (2)

To improve aerodynamic performance, the dimple surface coverage (SR),defined as the sum of the surface areas on the surface of a hypotheticalsphere that are circumscribed by the edges of the respective dimples asa proportion of the surface area of the hypothetical sphere, is set toat least 70%. SR may be set to preferably at least 71%, and morepreferably at least 72%.

Dimple Parameter (3)

To improve the aerodynamic performance, the dimple volume ratio (VR),defined as the sum of the volumes of individual dimple spaces below aflat plane circumscribed by the edge of each dimple on a golf ball as aproportion of the volume of the golf ball were it to have no dimples onthe surface (hypothetical sphere), is set to at least 1.06%. It isrecommended that VR be set to preferably at least 1.1%, more preferablyat least 1.15%, and even more preferably at least 1.2%. The upper limitis not more than 1.5%, preferably not more than 1.4%, and morepreferably not more than 1.3%.

Dimple Parameter (4)

The number of dimple types, i.e., types of dimples of mutually differingdiameter DM and/or depth DP, is set to three or more. The number oftypes may be set to preferably at least four, and more preferably atleast five. The upper limit is preferably not more than 14 types, andmore preferably not more than 10 types. The number of types of dimplesis selected as appropriate in this way so as to facilitate an increasein the surface coverage SR specified in the invention.

Here, referring to FIG. 2, the depth DP of a dimple is the verticaldistance from a hypothetical flat plane L, traced by connecting thepositions where the dimple meets land areas, to the bottom (deepestposition) of the dimple. In addition, as shown in FIG. 2, the diameterDM of a dimple is the diameter (span) between positions where the dimpleportion is tangent with land areas (non-dimple forming portions), i.e.,between the high points of the dimple portion. In most cases, the golfball is subjected to surface treatment such as painting. In such balls,the dimple diameter and depth refer to the diameter and depth after thecoat of paint has been applied.

Dimple Parameter (5)

To obtain a proper trajectory, the average dimple depth is set to atleast about 0.18 mm. It is recommended that the average dimple depth beset to preferably at least about 0.19. The upper limit is preferably notmore than about 1.0 mm, more preferably not more than about 0.7 mm, andeven more preferably not more than about 0.5 mm. Here, “average dimpledepth” refers to the average of the depths DP of all the dimples.

The average dimple diameter DM, while not subject to any particularlimitation, is preferably at least about 3.0 mm, more preferably atleast about 3.2 mm, and even more preferably at least about 3.5 mm. Theupper limit is preferably not more than about 7.5 mm, more preferablynot more than about 6.5 mm, and even more preferably not more than about6 mm. Here, “average dimple diameter DM” refers to the average of thediameters of all the dimples.

Dimple Parameter (6)

The ratio of the dimple diameter DM to the dimple depth DP, or DM/DP,has an average value of not more than about 23. It is recommended thatthis average value be preferably not more than about 22, more preferablynot more than about 21, and even more preferably not more than about 20.The lower limit, while not subject to any particular limitation, ispreferably at least about 5, more preferably at least about 8, and evenmore preferably at least about 10.

Dimple Parameter (7)

In the present invention, although not subject to any particularlimitation, when the dimples are divided into dimples Da having adiameter of 3.7 mm or more and smaller dimples Db, the (total number ofDa dimples)/(total number of Db dimples) ratio is preferably set to atleast about 0.005 but not more than about 1. The lower limit is morepreferably at least about 0.01, even more preferably at least about 0.1,still more preferably at least about 0.2, and most preferably at leastabout 0.3. The upper limit is more preferably not more than about 0.8,even more preferably not more than about 0.6, and most preferably notmore than about 0.5.

The dimples Da having a diameter of at least 3.7 mm account for aproportion of the total dimple volume which, while not subject to anyparticular limitation, is preferably at least about 75%, more preferablyat least about 78%, and even more preferably at least about 80%. Theupper limit value is preferably not more than about 98%, more preferablynot more than about 95%, and even more preferably not more than about92%.

The average diameter (Dm) of the Da dimples is preferably at least about3.7 mm, and more preferably at least about 3.8 mm. The upper limitthereof is preferably not more than about 7 mm, and more preferably notmore than about 6 mm. The average depth (Dp) of the Da dimples ispreferably at least about 0.05 mm, and more preferably at least about0.1 mm. The upper limit thereof is preferably not more than about 0.5mm, and more preferably not more than about 0.3 mm. The average volumeof the Da dimples is preferably at least about 0.8 mm³, and morepreferably at least about 1.0 mm³. The upper limit thereof is preferablynot more than about 3.0 mm³, and more preferably not more than about 2.5mm³. The ratio Dm/Dp for the Da dimples is preferably at least about 7,and more preferably at least about 8, and the upper limit thereof ispreferably not more than about 25, and more preferably not more thanabout 23. If the above numerical value ranges are not satisfied, the lowtrajectory that is desired may not be obtained, which may make itimpossible to achieve the objects of the invention.

The average diameter (Dm) of the Db dimples is preferably at least about1 mm, and more preferably at least about 2 mm. The upper limit is lessthan about 3.7 mm, and more preferably not more than about 3.5 mm. Theaverage depth (Dp) of the Db dimples is preferably at least about 0.05mm, and more preferably at least about 0.1 mm. The upper limit thereofis preferably not more than about 0.3 mm, and more preferably not morethan about 0.2 mm. The average volume of the Db dimples is preferably atleast about 0.2 mm³, and more preferably at least about 0.3 mm³. Theupper limit thereof is preferably not more than about 1.5 mm³, and morepreferably not more than about 1.0 mm³. The ratio Dm/Dp for the Dbdimples is preferably at least about 10, and more preferably at leastabout 12. The upper limit thereof is preferably not more than about 30,and more preferably not more than about 26. If the above numerical valueranges are not satisfied, the low trajectory that is desired may not beobtained, which may make it impossible to achieve the objects of theinvention.

Dimple Parameter (8)

To improve the distance a golf ball travels, it is desirable for theball to have a low coefficient of drag (CD) under high-velocityconditions and a high coefficient of lift (CL) under low-velocityconditions. Thus, in the present invention, with regard to thelow-velocity CL, it is critical for the coefficient of lift CL when theball is launched using an Ultra Ball Launcher (UBL) at a Reynolds numberof 70,000 and a spin rate of 2,000 rpm to be maintained at 60% or more,and preferably at 65% or more, of the coefficient of lift CL when theball is launched at a Reynolds number of 80,000 and a spin rate of 2,000rpm.

Dimple Parameter (9)

The dimples have an average edge angle of preferably at least 11degrees, more preferably at least 12 degrees, and even more preferablyat least 13 degrees. The upper limit is preferably not more than 17degrees, more preferably not more than 16 degrees, and even morepreferably not more than 15 degrees. If the average edge angle is toolarge, the trajectory may become too low, possibly resulting in toolarge a difference with a customary trajectory. On the other hand, ifthe average edge angle is too small, it may not be possible to obtainthe effect of holding down the decrease in distance traveled by the ballon shots at a low HS while reducing the distance traveled on shots takenat a high HS. As used herein, “average edge angle” refers to the averageedge angle for all the dimples.

Dimple Parameter (10)

It is recommended that the proportion of dimples having an edge angle offrom 12 to 16 degrees be preferably more than 70%, more preferably atleast 80%, and even more preferably at least 90%, of the total number ofdimples formed on the ball surface. If the proportion of such dimples istoo small, it may not be possible to obtain the effect of holding downthe decrease in distance traveled by the ball on shots at a low HS whilereducing the distance traveled on shots taken at a high HS.

The edge angle of a dimple is defined herein as follows. Referring toFIG. 9, let us imagine over the dimple D a first spherical surface(i.e., the spherical surface of the golf ball were it to have no dimplesthereon) Q₁ prior to formation of the dimple. Let us also imagine asecond spherical surface Q₂ which is centered at the center point of thegolf ball and has a radius 0.04 mm smaller than that of the firstspherical surface Q₁. If we then draw tangents T and T′ at points P andP′ where the second spherical surface Q₂ intersects the wall of thedimple D, the points E and E′ where the tangents T and T′ intersect thefirst spherical surface Q₁ represent the respective edges of the dimpleD. The angle θ between the line segment (straight line) L connectingpoints E and E′ determined in this way and the tangents T and T′ is theedge angle.

The shapes of the dimples are not limited to circular shapes, and mayalso be suitably selected from among, for example, polygonal,tear-shaped and oval shapes. Setting the number of dimple types to atleast three, and preferably at least five, makes it possible for thedimples to cover a higher proportion of the spherical surface. Also, byinterspersing large and small dimples, the surface coverage can beincreased to the specified range. Because this enables extremefluctuations in the coefficient of lift (CL) within the low-velocityregion to be suppressed, the ball can be given a relatively lowtrajectory, making it easier to elicit the advantageous effects of theinvention.

The golf ball of the invention can be made to conform with the Rules ofGolf for competitive play, and may be formed to a diameter of not lessthan 42.67 mm. It is suitable to set the weight to generally not lessthan 45.0 g, and preferably not less than 45.2 g, but not more than45.93 g.

As described above, in this invention, it is possible to substantiallyreduce the distance traveled by the ball on high HS shots while at thesame time holding down as much as possible the decrease in distancetraveled on low HS shots. As a result, a superior golf ball forcompetitors having a low head speed can be obtained.

EXAMPLES

The following Examples and Comparative Examples are provided by way ofillustration and not by way of limitation.

Examples 1 to 5, Comparative Examples 1 to 5

The rubber compositions shown in Table 1 were prepared, then molded andvulcanized at 155° C. for 15 minutes to produce solid cores.

TABLE 1 A B C D E Formulation Polybutadiene rubber (1) 100 100 100 100(parts by Polybutadiene rubber (2) 100 weight) Zinc acrylate 25.5 28.033.5 38.0 22.5 Peroxide (1) 0.6 0.6 1.1 1.1 0.6 Peroxide (2) 0.6 0.6 0.6Zinc oxide 4 4 4 4 4 Barium sulfate 30.9 14.5 28 19.3 29.1 Calciumcarbonate 12 2 5 Zinc stearate 5 5 5 5 Antioxidant 0.1 0.1 0.2 0.2 0.1Zinc salt of 1 1 pentachlorothiophenol Sulfur 0.09 0.09 Specific gravity1.226 1.196 1.226 1.196 1.226

Trade names of the materials in the table are as follows.

-   Polybutadiene rubber (1): Available under the trade name “BR 01”    from JSR Corporation.-   Polybutadiene rubber (2): Available under the trade name “BR 730”    from JSR Corporation.-   Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd.-   Peroxide (1): Dicumyl peroxide, available under the trade name    “Percumyl D” from NOF Corporation.-   Peroxide (2): 1,1-Bis(t-butylperoxy)cyclohexane; available under the    trade name “Perhexa C-40” from NOF Corporation.-   Zinc oxide: Available from Sakai Chemical Industry Co., Ltd.-   Zinc stearate: Available under the trade name “Zinc Stearate G” from    NOF Corporation.-   Barium sulfate: Available under the trade name “Precipitated Barium    Sulfate 100” from Sakai Chemical Industry Co., Ltd.-   Calcium carbonate: Available under the trade name “Silver W” from    Shiraishi Calcium Kaisha, Ltd.-   Antioxidant: Available under the trade name “Nocrac NS-6” from Ouchi    Shinko Chemical Industry Co., Ltd.

Next, the cover material shown in Table 2 below was injection moldedover the above core, thereby obtaining a multi-piece solid golf ball inwhich the core is encased by an inner cover layer and an outer coverlayer of given thicknesses.

TABLE 2 1 2 3 4 Formulation4 Himilan 1557 42.5 50 (parts by weight)Himilan 1601 42.5 50 Himilan 1605 69 Nucrel AN4318 15 Nucrel AN4319 84Nucrel 1560 1 Dynaron 6100P 15 31 Titanium oxide 4.8 2.8 Calciumhydroxide 2.3 Polytail H 2 Behenic acid 18 Magnesium oxide 1 Magnesiumstearate 59

Trade names of the materials in the table are as follows.

-   Himilan: Ionomeric resins available from DuPont-Mitsui Polychemicals    Co., Ltd.-   Nucrel AN4318, AN4319: Terpolymers available from DuPont-Mitsui    Polychemicals Co., Ltd.-   Nucrel 1560: A copolymer available from DuPont-Mitsui Polychemicals    Co., Ltd.-   Dynaron 6100P: A hydrogenated polymer available from JSR    Corporation.-   Titanium oxide: Available under the trade name “Tipaque R550” from    Ishihara Sangyo Kaisha, Ltd.-   Calcium hydroxide: Available under the trade name “CLS-B” from    Shiraishi Calcium Kaisha, Ltd.-   Polytail H: A low-molecular-weight polyolefin polyol available from    Mitsubishi Chemical Corporation.-   Behenic acid: Available under the trade name “NAA-222S” from NOF    Corporation.-   Magnesium oxide: Available as “Kyowamag MF150” from Kyowa Chemical    Industry Co., Ltd.-   Magnesium stearate: Available under the trade name “Magnesium    Stearate G” from NOF Corporation.

Numerous dimples were formed on the surface of the cover simultaneouswith injection molding of the cover, after which the cover wasspray-painted. In each example and comparative example, the dimples onthe surface of the ball after painting satisfied the parameters shown inTables 3 to 8 below. In these tables, the dimple types designated as Darefer to dimples having a diameter of 3.7 mm or more, and the dimpletypes designated as Db refer to dimples having a diameter of less than3.7 mm.

With regard to the dimple patterns in the tables, the dimple pattern forExamples 1, 3 and 5 is shown in Table 3 (FIG. 3), the pattern forExamples 2 and 4 is shown in Table 4 (FIG. 4), the pattern forComparative Examples 1 and 5 is shown in Table 5 (FIG. 5), the patternfor Comparative Example 2 is shown in Table 6 (FIG. 6), the pattern forComparative Example 3 is shown in Table 7 (FIG. 7), and the pattern forComparative Example 4 is shown in Table 8 (FIG. 8). These diagrams areall top views of the ball. In the respective examples, the bottom viewsof the ball have the same pattern as the top views, and are thusomitted.

TABLE 3 Number Edge Examples 1, 3 and 5 of Diameter Depth Volume angleDimple types dimples (mm) (mm) (mm³) (°) Da-I 40 4.1 0.21 1.53 14.0Da-II 184 3.9 0.20 1.31 14.0 Db-I 96 3.3 0.16 0.73 14.2 Da-III 32 4.10.23 1.72 15.6 Da-IV 16 3.9 0.22 1.45 15.4 Db-II 16 3.2 0.15 0.62 13.5Db-III 8 3.2 0.14 0.49 11.3

TABLE 4 Number Edge Examples 2 and 4 of Diameter Depth Volume angleDimple types dimples (mm) (mm) (mm³) (°) Da-I 24 4.5 0.20 1.66 12.3Da-II 150 4.3 0.19 1.48 12.0 Da-III 66 3.7 0.18 1.02 12.3 Db-I 18 2.70.13 0.41 12.4 Db-II 6 2.5 0.12 0.31 12.2 Da-IV 48 4.3 0.19 1.56 13.7Da-V 12 3.8 0.18 1.15 14.6 Db-III 6 3.4 0.16 0.75 11.8 Db-IV 6 3.3 0.150.66 11.4

TABLE 5 Comparative Examples Number Edge 1 and 5 of Diameter DepthVolume angle Dimple types dimples (mm) (mm) (mm³) (°) Da-I 24 4.7 0.151.25 9.8 Da-II 168 4.5 0.15 1.15 9.4 Da-III 48 3.9 0.15 0.85 10.3 Db-I12 2.9 0.15 0.44 13.3 Db-II 12 2.6 0.11 0.24 11.6 Da-IV 30 4.4 0.16 1.2010.2 Da-V 36 3.9 0.17 0.94 11.3 Db-III 8 3.5 0.16 0.70 11.8 Db-IV 6 3.40.15 0.61 11.4

TABLE 6 Number Edge Comparative Example 2 of Diameter Depth Volume angleDimple types dimples (mm) (mm) (mm³) (°) Da-I 12 4.6 0.16 1.28 10.0Da-II 222 4.4 0.16 1.16 9.9 Da-III 36 3.8 0.15 0.80 10.4 Db-I 12 2.60.12 0.58 12.0 Da-IV 12 4.4 0.17 0.25 10.6 Da-V 24 3.8 0.16 1.25 11.0Db-II 6 3.5 0.16 0.86 11.8 Db-III 6 3.4 0.15 0.70 11.4

TABLE 7 Number Edge Comparative Example 3 of Diameter Depth Volume angleDimple types dimples (mm) (mm) (mm³) (°) Da-I 228 4.3 0.17 1.06 11.5Da-II 36 3.7 0.16 0.74 11.0 Db-I 12 2.5 0.12 0.23 12.2 Db-II 12 3.4 0.170.72 12.6 Da-III 42 4.3 0.18 1.14 12.2 Da-IV 24 3.7 0.17 0.80 11.8 Da-V12 4.3 0.17 1.05 10.6 Da-VI 2 3.9 0.16 0.89 10.5

TABLE 8 Number Edge Comparative Example 4 of Diameter Depth Volume angleDimple types dimples (mm) (mm) (mm³) (°) Db-I 114 3.65 0.20 1.07 13.3Da-I 114 4.0 0.15 1.01 10.4 Db-II 60 3.65 0.20 1.07 13.3 Db-III 12 2.50.17 0.43 16.1 Da-II 60 4.0 0.15 1.01 10.4

Various properties of the resulting multi-piece solid golf balls wereinvestigated as described below. The results are shown in Tables 9 and10.

Center Hardness of Solid Core, Cross-Sectional Hardness Midway BetweenCenter and Surface of Core, and Surface Hardness of Core

The cross-sectional hardness (Shore D hardness) of the solid core wasmeasured by cutting the core through the center, and perpendicularlypressing the indenter of a type D durometer conforming with ASTMD2240-95 against the center of the cross-section, and at a positionmidway between the center and surface of the cross-section.

The surface hardness (Shore D hardness) of the solid core was measuredby perpendicularly pressing the indenter of a type D durometerconforming with ASTM D2240-95 against the spherical surface of the core.

The above hardnesses are each measured values obtained after holding thecore isothermally at 23° C.

Deflection of Solid Core, Inner Cover Layer-Encased Sphere, and Ball

The solid core, inner cover layer-encased sphere and ball were placed ona hard plate, and the amount of deflection by each when compressed undera final load of 1,275 N (130 kgf) from an initial load state of 98 N (10kgf) was measured. In addition, the deflection by the ball whencompressed under a final load of 5,880 N (600 kgf) from an initial loadstate of 98 N (10 kgf) was similarly measured.

The above deflections are each measured values obtained after holdingthe specimen to be measured isothermally at 23° C.

Cover Hardness (Shore D Hardness)

The cover-forming material was formed under applied pressure to athickness of about 2 mm and the resulting sheet was held at 23° C. for 2weeks, following which the Shore D hardness of the sheet was measured inaccordance with ASTM D2240.

CL Ratio

The ratio of the coefficient of lift CL of a ball launched using anUltra Ball Launcher (UBL) at a Reynolds number of 70,000 and a spin rateof 2,000 rpm with respect to the coefficient of lift CL of a balllaunched at a Reynolds number of 80,000 and a spin rate of 2,000 rpm wascalculated.

Initial Velocity

The initial velocity of the ball was measured using an initial velocitymeasuring apparatus of the same type as the USGA drum rotation-typeinitial velocity instrument approved by the R&A. The ball was heldisothermally in a 23±1° C. environment for at least 3 hours, then testedin a chamber at a room temperature of 23±2° C. The ball was hit using a250-pound (113.4 kg) head (striking mass) at an impact velocity of 143.8ft/s (43.83 m/s). One dozen balls were each hit four times. The timetaken for the ball to traverse a distance of 6.28 ft (1.91 m) wasmeasured and used to compute the initial velocity (m/s) of the ball.This cycle was carried out over a period of about 15 minutes.

Flight Performance

A driver (W#1) was mounted on a swing robot, and the distance traveledby the ball when hit at a head speed (HS) of 54 m/s or 35 m/s wasmeasured. The club used was a TOURSTAGE X-DRIVE 701 (2009 model; loftangle, 9.5°) manufactured by Bridgestone Sports Co., Ltd.

The flight performance was rated according to the following criteria.

-   -   Good: The difference in total distance between shots taken at a        head speed of 54 m/s and shots taken at a head speed of 35 m/s        was less than 97 m    -   NG: The difference in total distance between shots taken at a        head speed of 54 m/s and shots taken at a head speed of 35 m/s        was 97 m or more

TABLE 9 Example 1 2 3 4 5 Core Formulation A A B C D Diameter (mm) 36.136.1 37.3 36.1 37.3 10-130 kgf deflection (mm) 4.2 4.2 3.4 4.3 3.5Center hardness Hc (Shore D) 36 36 41 34 37 Cross-sectional hardness Hmmidway 41 41 46 38 41 between center and surface (Shore D) Surfacehardness Hs (Shore D) 51 51 56 55 59 Inner cover Material 1 1 2 1 2layer Material hardness (Shore D) 49 49 56 49 56 Thickness (mm) 1.951.95 1.35 1.95 1.35 Inner cover 10-130 kgf deflection (mm) 3.7 3.7 3.03.7 3.0 layer-encased sphere Outer cover Material 3 3 4 3 4 layerMaterial hardness (Shore D) 57 57 61 57 61 Thickness (mm) 1.35 1.35 1.351.35 1.35 Dimples Number of dimple types 7 types 9 types 7 types 9 types7 types Number of dimples 392 336 392 336 392 SR value (%) 72 76 72 7672 VR value (%) 1.20 1.06 1.20 1.06 1.20 Average DP (mm) 0.19 0.18 0.190.18 0.19 Average edge angle (°) 14.2 12.4 14.2 12.4 14.2 Proportion ofdimples having an 98 96 98 96 98 edge angle of 12 to 16° (%) AverageDM/DP 19.83 21.95 19.83 21.95 19.83 (Total number of Db dimples)/ 0.440.12 0.44 0.12 0.44 (Total number of Da dimples) Volume proportion of Dadimples (%) 82 96 82 96 82 Low-velocity CL ratio (%) 85 78 85 78 85 BallDiameter (mm) 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.3 45.3 45.3 45.345.3 10-130 kgf deflection (mm) 3.2 3.2 2.6 3.2 2.6 10-600 kgfdeflection (mm) 10.2 10.2 8.7 11.1 9.4 Initial velocity (m/s) 77.3 77.376.8 77.3 76.8 Hardness Hm − Hc (Shore D) 5 5 5 4 4 relationship Hs − Hm(Shore D) 10 10 10 17 18 Hs − Hc (Shore D) 15 15 15 21 22 Materialhardness of outer cover 8 8 5 8 5 layer − Material hardness of innercover layer (Shore D) Hs - Material hardness of 2 2 0 6 3 inner coverlayer (Shore D) Hs - Material hardness of −6 −6 −5 −2 −2 outer coverlayer (Shore D) Deflection Intermediate layer-encased sphere/Core 0.880.88 0.88 0.86 0.86 ratio Ball/Core 0.76 0.76 0.76 0.74 0.74 (600 kgfball deflection)/(130 3.2 3.2 3.3 3.5 3.6 kgf ball deflection) FlightHS, 54 m/s Carry (m) 259.1 263.5 263.1 264.3 264.3 Total distance (m)274.0 276.4 276.1 277.2 277.6 HS, 35 m/s Carry (m) 161.9 162.0 163.1162.3 163.6 Total distance (m) 182.6 180.5 181.6 181.2 182.4 Differencein carry (m) 97.2 101.5 100.0 102.0 100.7 Difference in total distance(m) 91.4 95.9 94.5 96.0 95.2 Rating good good good good good

TABLE 10 Comparative Example 1 2 3 4 5 Core Formulation E E E E ADiameter (mm) 36.1 36.1 36.1 36.1 36.1 10-130 kgf deflection (mm) 4.24.2 4.2 4.2 4.2 Center hardness Hc (Shore D) 36 36 36 36 36Cross-sectional hardness Hm midway 41 41 41 41 41 between center andsurface (Shore D) Surface hardness Hs (Shore D) 51 51 51 51 51 Innercover Material 1 1 1 1 1 layer Material hardness (Shore D) 49 49 49 4949 Thickness (mm) 1.95 1.95 1.95 1.95 1.95 Inner cover 10-130 kgfdeflection (mm) 3.7 3.7 3.7 3.7 3.7 layer-encased sphere Outer coverMaterial 3 3 3 3 3 layer Material hardness (Shore D) 57 57 57 57 57Thickness (mm) 1.35 1.35 1.35 1.35 1.35 Dimples Number of dimple types 9types 8 types 8 types 5 types 9 types Number of dimples 344 330 368 360344 SR value (%) 80 78 76 71 80 VR value (%) 0.90 0.88 0.93 0.90 0.90Average DP (mm) 0.15 0.15 0.16 0.17 0.15 Average edge angle (°) 10.110.2 11.6 12.0 10.1 Proportion of dimples having an 3 4 18 52 3 edgeangle of 12 to 16° (%) Average DM/DP 27.39 24.77 23.17 20.97 27.39(Total number of Db dimples)/ 0.12 0.078 0.07 1.07 0.12 (Total number ofDa dimples) Volume proportion of Da dimples (%) 95 95 97 48 95Low-velocity CL ratio (%) 80 78 65 75 80 Ball Diameter (mm) 42.7 42.742.7 42.7 42.7 Weight (g) 45.3 45.3 45.3 45.3 45.3 10-130 kgf deflection(mm) 3.2 3.2 3.2 3.2 3.2 10-600 kgf deflection (mm) 10.2 10.2 10.2 10.210.2 Initial velocity (m/s) 76.3 76.3 76.3 76.3 77.3 Hardness Hm − Hc(Shore D) 5 5 5 5 5 relationship Hs − Hm (Shore D) 10 10 10 10 10 Hs −Hc (Shore D) 15 15 15 15 15 Material hardness of outer cover 8 8 8 8 8layer − Material hardness of inner cover layer (Shore D) Hs - Materialhardness of 2 2 2 2 2 inner cover layer (Shore D) Hs - Material hardnessof −6 −6 −6 −6 −6 outer cover layer (Shore D) Deflection Intermediatelayer-encased sphere/Core 0.88 0.88 0.88 0.88 0.88 ratio Ball/Core 0.760.76 0.76 0.76 0.76 (600 kgf ball deflection)/(130 3.2 3.2 3.2 3.2 3.2kgf ball deflection) Flight HS, 54 m/s Carry (m) 264.2 265.0 263.5 263.2270.8 Total distance (m) 275.9 276.1 274.5 274.1 282.6 HS, 35 m/s Carry(m) 157.8 156.8 157.7 157.0 160.9 Total distance (m) 175.4 174.9 176.0175.5 178.1 Difference in carry (m) 106.4 108.2 105.8 106.2 109.9Difference in total distance (m) 100.5 101.2 98.5 98.6 104.5 Rating NGNG NG NG NG

In the above table, Comparative Examples 1 to 4 are prior-artreduced-distance golf balls, and Comparative Example 5 is a prior-arthigh-rebound golf ball. Here, on comparing the reduced-distance golfballs of Examples 1 to 5 with those of Comparative Examples 1 to 4, itcan be seen that the balls in Comparative Examples 1 to 4, owing totheir lower rebound (initial velocity) relative to the prior-arthigh-rebound golf ball of Comparative Example 5, travel substantiallyreduced distances (both the carry and the total distance) not only at ahigh head speed but also at a low head speed. By contrast, it wasconfirmed that the golf balls in Examples 1 to 5 of the invention, byhaving the same rebound (initial velocity) as the high-rebound golf ballin Comparative Example 5 and by combining therewith dimples whichsatisfy specific parameters and can thus achieve a relatively lowtrajectory, are more effective than the balls in Comparative Examples 1to 4 at suppressing the decrease in distance when hit at a low headspeed relative to the substantial reduction in distance when hit at ahigh head speed. That is, the golf balls in the examples according tothe present invention were confirmed to be golf balls which have a smalldifference in distance when hit at a high head speed versus when hit ata low head speed, and which are thus able to achieve a superior distancein the low head speed range while holding down the distance traveled inthe high head speed range.

1. A multi-piece solid golf ball comprising a solid core, an inner coverlayer and an outer cover layer, which outer cover layer has numerousdimples on a surface thereof, wherein the inner cover layer has athickness of from 0.8 to 3.0 mm and a material hardness, in terms ofShore D hardness, of from 10 to 60, the outer cover layer has athickness of from 0.7 to 3.0 mm and a material hardness, in terms ofShore D hardness, of from 45 to 62, and the material hardness of theouter cover layer is higher than the material hardness of the innercover layer; the dimples number at least 250 but not more than 50%, havea surface coverage (SR) of at least 70% and a volume ratio (VR) of atleast 1.06%, are of at least three types of mutually differing dimplediameter (DM) and/or dimple depth (DP), and have an average depth of atleast about 0.18 mm and an average diameter-to-depth ratio (DM/DP) ofnot more than about 23; and the ball has a coefficient of lift CL at aReynolds number of 70,000 and a spin rate of 2,000 rpm which ismaintained at 60% or more of a coefficient of lift CL at a Reynoldsnumber of 80,000 and a spin rate of 2,000 rpm.
 2. The multi-piece solidgolf ball of claim 1 wherein, letting Da represent dimples having adiameter of at least 3.7 mm and Db represent dimples having a diameterof less than 3.7 mm, the ratio (total number of Db dimples)/(totalnumber of Da dimples) is at least about 0.005 but not more than about 1.3. The multi-piece solid golf ball of claim 2, wherein the dimples Dahaving a diameter of at least 3.7 mm account for at least about 75% ofthe total dimple volume.
 4. The multi-piece solid golf ball of claim 1,wherein the value obtained by subtracting the material hardness of theinner cover layer from the material hardness of the outer cover layer(outer cover layer material hardness−inner cover layer materialhardness) is, in terms of Shore D hardness, at least 5 but not more than50.
 5. The multi-piece solid golf ball of claim 1, wherein the dimpleshave an average edge angle of from 11 to 17 degrees.
 6. The multi-piecesolid golf ball of claim 1, wherein the proportion of dimples having anedge angle of from 12 to 16 degrees is more than 70% of the total numberof dimples formed on the surface of the ball.
 7. The multi-piece solidgolf ball of claim 1, wherein the value obtained by subtracting theinner cover layer material hardness from a surface hardness (Hs) of thecore (Hs−inner cover layer material hardness) is, in terms of Shore Dhardness, greater than −10 and less than +10.
 8. The multi-piece solidgolf ball of claim 1, wherein the value obtained by subtracting theouter cover layer material hardness from a surface hardness (Hs) of thecore (Hs−outer cover layer material hardness) is, in terms of Shore Dhardness, at least −15 and not higher than +5.
 9. The multi-piece solidgolf ball of claim 1, wherein the ratio of deflection by a spherecomposed of the solid core encased by the inner cover layer (inner coverlayer-encased sphere) when compressed under a final load of 1,275 N (130kgf) from an initial load state of 98 N (10 kgf) to deflection by thesolid core when compressed under a final load of 1,275 N (130 kgf) froman initial load state of 98 N (10 kgf), which ratio is represented as(inner cover layer-encased sphere deflection)/(solid core deflection),is from 0.82 to 0.92.
 10. The multi-piece solid golf ball of claim 1,wherein the ratio of deflection by the ball when compressed under afinal load of 1,275 N (130 kgf) from an initial load state of 98 N (10kgf) to deflection by the solid core when compressed under a final loadof 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), whichratio is represented as (ball deflection)/(solid core deflection), isfrom 0.72 to 0.79.
 11. The multi-piece solid golf ball of claim 1,wherein the ratio of deflection by the ball when compressed under afinal load of 5,880 N (600 kgf) from an initial load state of 98 N (10kgf) to deflection by the ball when compressed under a final load of1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), whichratio is represented as (600 kgf deflection/130 kgf deflection), is from3.2 to 3.7.
 12. The multi-piece solid golf ball of claim 1, wherein thecore has a center hardness (Hc), a surface hardness (Hs) and across-sectional hardness (Hm) at an intermediate position between thecore center and the core surface which, in terms of Shore D hardnesses,satisfy the following conditions:Hm−Hc=0 to 7,Hs−Hm=11 to 25, andHs−Hc≧16.
 13. The multi-piece solid golf ball of claim 1, wherein thecore has a center hardness (Hc), a surface hardness (Hs) and across-sectional hardness (Hm) at an intermediate position between thecore center and the core surface which, in terms of Shore D hardnesses,satisfy the following condition:Hs−Hm>(Hm−Hc)×3.
 14. The multi-piece solid golf ball of claim 1 whereindimples Da with a diameter of at least 3.7 mm have an average diameterof at least 3.7 mm but not more than 6 mm, and dimples Db with adiameter of less than 3.7 mm have an average diameter of at least 1 mmbut less than 3.7 mm.
 15. The multi-piece solid golf ball of claim 1,wherein dimples Da with a diameter of at least 3.7 mm have an averagedepth of from 0.05 to 0.5 mm, and dimples Db with a diameter of lessthan 3.7 mm have an average depth of from 0.05 to 0.3 mm.
 16. Themulti-piece solid golf ball of claim 1, wherein dimples Da with adiameter of at least 3.7 mm have an average volume of from 0.8 to 3.0mm³, and dimples Db with a diameter of less than 3.7 mm have an averagevolume of from 0.2 to 1.5 mm³.
 17. The multi-piece solid golf ball ofclaim 1, wherein dimples Da with a diameter of at least 3.7 mm have anaverage diameter (Dm) to average depth (Dp) ratio Dm/Dp of from 7 to 25,and dimples Db with a diameter of less than 3.7 mm have an averagediameter (Dm) to average depth (Dp) ratio Dm/Dp of from 10 to
 30. 18.The multi-piece solid golf ball of claim 1, wherein the cover is formedof a material comprising: (A) a thermoplastic polyurethane material, and(B) an isocyanate mixture obtained by dispersing (B-1) an isocyanatecompound having as functional groups at least two isocyanate groups permolecule in (B-2) a thermoplastic resin that is substantiallynon-reactive with isocyanate.
 19. The multi-piece solid golf ball ofclaim 1, wherein the cover is formed of a material comprising: (D) athermoplastic polyurethane, and (E) a polyisocyanate compound.